So many misconceptions.
They recommend " cowboy loads" which are a lot slower than full power magnum loads.
BP guns are not proofed for use with smokeless powder.
The cylinder is the pressure vessel in a revolver, not the barrel or the frame. If anything is going to blow, it will be the cylinder because the cross sections of the cylinder geometry are thinner than anywhere else. Reputable conversion cylinders are made of 4150 and 4140 arsenal steel and are proofed for SAAMI spec 45 Colt loads.
I have a conversion cylinder for my ROA's. I don't use it much because it's a lot of hassle to reload. It's neat to have around but it's kind of like holding a hand grenade. And just think of all the work that goes into reloading; you have to clean the brass, resize & decap, clean each primer pocket (or re-tumble again), depending on the cartridge you may have to check case length and shave some off, weigh-up the charges, and seat the bullet.
You don't have clean the primer pockets separately or tumble again. I just rinse out my BP brass and then tumble it clean when it is dry. And I have never, ever had to trim revolver brass, particularly not when loaded with Black Powder. The pressure developed is not high enough to stretch the brass. I have measured it. One of these days I have to throw away that useless case trimmer that is taking up extra room on my bench. If you don't reload, fine. But reloading for Black Powder is not a whole lot different than reloading for Smokeless.
Well, I load .45ACP AND .45 Colt, but the Colt gets loaded on an old Lyman turret where the .45ACP is loaded on a progressive. 15 minutes will knock out a box of .45ACP and that's being slow and careful.
Reason I don't consider .45 Colt is I have a 4 5/8" Ruger Blackhawk in the caliber and I don't think I'd wanna get confused even with my "light loads" for that gun (8.3 grains Unique/255 cast flat nose) and load 'em in the Remmy, a bit much. I'd load BP in colt cases with the colt cylinder. However, my 5.0 grain B'eye/200 SWC standard loads for my Ruger KP90DC would be light enough in the Remmy. I'm kinda swinging back to getting a conversion cylinder. I need to go out in the shop and get an empty .45ACP case and see how much fffg it'll hold. I've got more .45ACP cases because I used to pick 'em up at the range. Now, my range is in my back yard. No more brass or backstop lead supply. Bummer.
I played with loading .45 Colt and 30 grains BP with cornmeal filler topped by a round ball shooting it in my Blackhawk. That was pretty fun and another reason why I don't want a .45 Colt cylinder, be kinda redundant. That Blackhawk is awesome accurate with about anything. It's stainless, so it's easier to clean up. It's kinda pretty with the engraving I had done to it and the Sanbar stag grips, but the BP didn't seem to tarnish anything. I had a stainless ROA for a while, never tarnished, usually cleaned it in the dishwasher. That gun got stolen and I replaced it with a blued ROA, but it's a good shooter.
I load my Black Power cartridges, 45 Colt, 45 Schofield, 44-40, 44 Russian, and 38-40 on my Hornady Lock and Load AP progressive press with a Lyman Black Powder measure on it. I can crank out BP loads pretty quick, almost as quick as Smokeless.
Labeling what is inside a box of ammo is a pretty good way to make sure the wrong ammo does not go into the wrong gun. Using different color boxes is another good way.
Stainless guns are not any easier to clean than blued guns. That is another misconception.
And I really wish guys would stop cleaning guns in the same dishwasher that cleans the plates their families eat of off.
That's another thing that confuses me. Pressure with a 200 grain bullet at 900 fps (my .45ACP load) is a lot lower than pressure with a 340 grain bullet at 1000 fps (a load I've played with in my Blackhawk). I don't understand the velocity limit thing for these cylinders. My utility .45 Colt load 8.3 grains Unique/255 cast flat nose pushes 950 fps from the 4 5/8" Blackhawk, but I think it might be a little much for the remmy. Too, fired from my 7" .45 Colt TC contender barrel, it's over 1000 fps. Sheesh......
Yes, you are absolutely correct on that score, velocity is no way to define how safe any ammo is, pressure is the only way to state it. For what it's worth, the pamphlet that came with one of my Remmie cylinders specifically states ".45 Colt "Cowboy Ammunition" is nationally available from a number of well known manufacturers. This ammunition is loaded for a maximum muzzle velocity of 750 to 850 fps., and these are the specifications we recommend." Unfortunately there is no official SAMMI spec for Cowboy ammo. (there is no official spec for +P 45 Colt either for that matter). The 1000 fps velocity that has been bandied about on this thread is the maximum velocity allowed in pistols for cowboy completion. It is written into the SASS rules as a guideline for handloaders, but it has nothing at all to do with what velocity spec ammo companies are using for Cowboy ammo. As I said earlier, these cylinder have been proofed for Sammi max loads.
I saved up and bought a Howell's conversion cylinder for my ROA. I shot about 25 shells through it (no safety notch so 5 is max.) and they didn't hit to the same POA. I haven't shot it since. It's not legal for BP hunting so it's essentially a toy that I grew tired of.
I probably wouldn't sell it just in case more governments like the present one get elected by the idiot voters.
Point of impact vs point of aim has nothing to do with the conversion cylinder. It has to do with the velocity and amount of recoil of the loads. Shooting a 45 Colt cartridge is pretty well guarantied to hit someplace else than a 160 or so grain round ball.
I have never seen the attraction of the BP conversion cylinders unless you don't have any cartridge revolvers, but I doubt many of us fall into that category. Presumably you got the BP revolver because you wanted to experience shooting a percussion revolver.
If you want a BP looking cartridge revolver, you can always get one of the colt open tops or factory made conversions. You can often find a used one for around 350 bucks. I have one myself, and that's what I would recommend over a conversion cylinder.
Yeah, as I stated earlier I bought my EuroArms Remmie in 1975. I shot it plenty as a Cap & Ball revolver. I bought it because I was tired of caps falling down into the action of my 44 caliber brass framed 'Navy'. But when the conversion cylinders became available I instantly saw the attraction of using a conversion cylinder in it for Cowboy Action shooting. I was already loading 45 Colt with Black Powder, that part was a no brainer. I bought the gun so long ago that I had long since amortized the investment. I don't even remember what it cost. The conversion cylinder cost $200 at the time. It was like buying a neat new gun for $200. The other one came along with the gun for a very reasonable price. Again, a no brainer and then I had a brace of Remmies with conversion cylinders because we always shoot two revolvers in CAS.
By the way, did I mention how much quicker and easier it is to clean a cartridge cylinder than a C&B cylinder? Particularly if you have several C&B cylinders.
I have bookmarked Mr. Hoof's site for future use depending on what I decide. I still thing if I get a conversion, it'll be .45ACP just because I sold my 1917 and don't have a revolver in the caliber, or that's yet another reason for the .45ACP. 
