As I've mentioned before, leading is related to bullet fit, bullet alloy, bullet hardness, the pressure of the load, barrel twist and the lube being used. Faster barrel twist and higher pressure at some point makes a gas check required but I've yet to find a magic threshold as there are other factors at play.
I've fired enough plain based bullets bhn ~13 loaded to ~40 kpsi @1200-1300 fps loads through revolvers to know that they can be just as accurate as gas checked gas bullets. But I'd also expect the same loads shot through a 1-12 or faster twist or cast from a softer alloy would require the use of a gas check.
I also have a 160 gr plain base bhn ~18 load that shoots as well as my 357 blackhawk can shoot that is likely clocking 1500 fps.
