It's been a while since this topic got batted around by us cats.
Jim explained it nicely here though, and I don't recall the military/civilian or big lock/refined lock differentiation. I have seen makers and vendors of locks recommend against lead, and some specifically stating that it would void any lock warranty.
Rawhide I don't recall coming up, but I'd want to "pre-clamp" that outside the lock to let most of the moisture dissipate and only bring it to the cock when nearly dry. But I have used water to soften veg-tanned leather to for best conformation and I've scraped/split thicker leather to make it the thickness desired.
Also, some might "hang" on the idea of "more sparks" as Daryl noted, but that doesn't factor in increased lock time. Sure you cannot detect it, but I'll bet Larry Pletcher's timing setup would detect a slower lock with PB in the jaws. When you change the mass of the cock, and the throw and spring do not change, it surely takes some microseconds longer to get "up to speed" and complete the throw.
It might even be as big a difference as priming powders make, pure speculation, but there's likely a difference. Probably be best to time that with a big-honkin' lock that is okay with Pb and simple see if it works faster with leather (unless you have lock-money to burn in the interest of science). There's a project for someone with access to the right equipment.
