"I reckin' velocity, size, an' hardness are all related, an' you gotta find the optimum level of each."
You're right. Bullet hardness, lubrication, and bore condition are the determining factors, and some faults can be overcome by adjusting one or the other for each particular firearm. Pressure and velocity are the by-products of the above. Bullet size is the foremost, before dealing with any of these other factors.
For a swaged bullet, you run out of usefullness once you pass the .38 Special pressures. They will definitely gum up the works if you push them at all fast.
I shoot air cooled WW's in my Sharps, and get no leading. Come to think of it, that's all I shoot in all calibers up to 1600fps. After that, I will water drop up to about 2000 fps. Then, I add a couple tablespoons of shot to the pot, oven temper, and have been able to run up to the 2600 fps. neighborhood with no leading at all. All the smokeless loads are with Felix lube, and BP is Emmerts' Lube.