Driftwood Johnson may have something to add to this thread.
For black powder revolvers:
Big Lube bullet:
First off, when you say "Black Powder Revolvers", I assume you are talking about Cap & Ball revolvers. The Big Lube bullet you have pictured is a bit unusual, it was designed specifically for C&B revolver. Notice the rebated diameter at the rear of the bullet. This is there so the bullet can be easily inserted into the mouth of a chamber of a C&B cylinder, without tipping. Once the bullet has been seated on the powder, the larger diameters both in front of the lube groove and behind the lube groove will seal the bullet in the chamber. These will also be the contact surfaces for the bullet in the rifling of the barrel. I believe the two rebated diameters at the rear of the bullet were designed so it could be used either in a 'standard' 44 caliber C&B, or the Ruger Old Army, which has a slightly larger chamber mouth diameter and groove diameter.
http://www.biglube.com/BulletMolds.aspx?ItemID=09d6fdda-c105-4c87-b269-68ebfdaba982
Now the disclaimer. I have never used any Big Lube bullets in a C&B revolver, only round balls.
All of my experience with Big Lube bullets has been in cartridges loaded with Black Powder. I currently load Big Lube bullets in 45 Colt, 45 Schofield, 44-40, 44 Russian, and 38-40.
Yes, the Big Lube bullets carry enough soft bullet lube to keep the bore of a rifle barrel coated with soft bullet lube for the entire length. The idea is you do not want to use modern, standard, hard bullet lubes with Black Powder. Using modern bullet lube with Black Powder can create hard, crusty fouling which fills the rifling grooves. Not only will this hard crusty fouling destroy accuracy, it is difficult to remove. Keeping the bore coated with soft BP compatible bullet lube, such as SPG, for its entire length will keep the fouling soft. Each bullet will wipe the bore clean of the lube left behind, and deposit a new layer of soft lube in the bore. That is the whole idea behind the Big Lube bullets, lots, and lots of lube.
A Lube Star was mentioned earlier. This is a lube star. It shows lube that was still on the bullet when it exited the muzzle. The star shaped pattern is a result of more lube exiting the muzzle where the rifling grooves are. A Lube Star is a good indication that you used enough soft lube on your bullets, and the entire bore has been coated with lube the entire length. Years ago, before I discovered Big Lube bullets, I was pan lubing regular hard cast bullets with a mixture of Bees Wax/Crisco. The single small lube groove on these bullets did not have enough soft lube on them to keep the barrel of a rifle lubed the entire length, the bore would get starved for lube about 6 inches from the muzzle. I regularly had to swab out the barrel of my rifle with a wet patch to keep the BP fouling from building up near the muzzle. I was loading 45 Colt and 44-40 with my pan lubed bullets in those days. Shooting the 45 Colt cartridges in a revolver, there was enough lube to keep the bore lubed the entire length. But my 44-40 rifle rounds left the bore of a rifle staved for lube near the muzzle. There are tricks for adding more lube to a cartridge loaded with bullets with skimpy lube grooves, but once I discovered Big Lube bullets I never looked back and have been using them ever since, probably close to 20 years now.
On the left in this photo is a 44-40 cartridge, on the right is a 45 Colt. Next to each cartridge is their respective Big Lube bullets, with and without lube. This demonstrates the huge amount of soft BP compatible bullet lube these bullets carry.
Here is a photo of the Big Lube bullets I use. Left to right: 38-40, 44 caliber for both 44-40 and 44 Russian, 45 caliber for 45 Schofield, and finally 45 Colt. The big bullet is not a Big Lube bullet, it is the bullet I use for 45-70. It has multiple grooves for soft bullet lube. All the way on the right is one of my old hard cast, pan lubed bullets I used to load in Black Powder cartridges. Notice how small the lube groove is, the reason this bullet did not carry enough soft lube to keep a rifle barrel lubed its entire length.
OK, back to the original questions.
Accuracy? I have no idea. I have never done much long range shooting with 44-40 or 38-40 in a rifle. And I have never used a bullet like the one you have pictured from Lee. All my BP cartridge shooting has been close up in CAS, long range accuracy is not needed. I have used that 45/70 bullet for some shooting at about 50 yards with my Trap Door rifle and I was hitting the target with every shot, but they were pretty big targets. I have not tried that bullet in my Sharps with its fancy sights for a long time, so I really do not know how well it would do at 100 yards or more, but I suspect it would do quite well.
Bottom line: You are shooting a revolver, not a rifle. I doubt super accuracy is really needed. So I really doubt it would make much difference which bullet you used. As long as you are sure to lube them with soft, Black Powder compatible bullet lube. I do suspect the Big Lube bullet might be easier to seat in a revolver chamber, because of the reduced diameters at the rear of the bullet. I have no idea how easy it is to seat that Lee bullet in a C&B chamber.
Lube wad? Absolutely unnecessary with a Big Lube bullet. The bullet caries plenty of lube, no need for more lube behind the bullet. The two wads your UK friend is talking about is one of the reasons I have been using Big Lube bullets for many years, and have never added any extra lube, lube cookies, dry wads, or anything else in all those years.