In my experience, which is not thst great, it depends a lot more on pressure than velocity. I am able to use eight grains of unique in my 629 without leading. Ten grains and it leads quickly. Using 4227 or 2400 at the same VELOCITY, and I get very little leading. Go up th same PRESURE with either of those powders, and leading is similar. The latest Lee manual describes this and there is something to it, although I don't know if it is the last word.
Also, for what it's worth, I believe that gas checks have more to do with scraping the leading out than they do with preventing leading in the first place. I may very well be wrong, as I don't know much. I do know that there is a very breif exposure to high temperatures on the bullet base, and that lead couldn't be melted by an acetylene torch in a second or two, but pressure will cause lead to"flow" like a liquid just as easy as temperature. It is something to think about.
Also, I have used plain old wheel bearing grease for lube many times, with very good results. I just wipe the base of the bullet off on a rag, and seat as usual. I know that grease will run when it is hot, but the bullet is a light press fit. I have never had problems with poweder contamination that I know of.