The conversion cylinders are bulletproof as long as you keep the pressures below 14000 PSI. I'm pretty sure they are proofed to standard pressure .45 Colt to ensure they don't blow in case somebody mistakenly shoots a modern .45 Colt.
The thing that was stressed to me when I called the manufacturer about that advertised "below 850 fps" threshold was to NOT use jacketed bullets. The frames are built to swage lead, not copper. With lead bullets, I was told you can push the fps up to 1000 and it's safe to shoot... and that's with smokeless.
Personally, I would not use any smokeless loaded lead bullets that go over 900 fps. I also would not use anything hard cast for everyday shooting. At speeds less than 900 fps, you don't need hardcast anyway.
To better answer this specific question, I would look back to what loads were being used in converted percussion revolvers. .45 Colt wasn't the predominant cartridge in these guns, there was .46 Rimfire, .44 Henry, .44-40, etc. The .44-40 was using, obviously, 40 grains of bp to push a 200 grain bullet, so I would suggest not shooting more than a 200 grain bullet when you want to shoot full power bp cartridges.
I think you could get away with shooting a 250 grain bullet with a max load of black powder, just not everyday.