It is alot of fun talking about these topics. I have a couple of experiences you may find interesting. I have a pet load of 40.0 grains of RL7, a cast 405 grain(straight WW), and Carnuba Red lube(50-50 mix with White lube). I shoot this load out of four different 45-70's. Two single shot's(Sharp's and 1885 High-Wall), and two lever actions(1895 Marlin, and 1886 Winchester). The only difference in the load is OAL, and crimping. I seat the two single shots out to the lands, I seat and crimp the other load shorter to function easier in the lever's. The load chrono's at 1600fps-give or take in all four rifles.
I get no leading, and great accuracy in the single shots. IMO there is very little deformation in the bullet. Again IMO it is due to no bullet jump, good sizing, and a good lube. Whe I shoot these longer cartridges in the lever's(they will chamber with maybe a little land's touching or the lands slightly pushing the bullet in a little farther-the bullet for my SS's are not crimped), they perform fantastic. The problem is I can only shoot them single shot style only. When I crimp the load and have some bullet jump, then I show some signs of leading, and accuracy is not as good as when I load the longer cartridge single shot style. This leads me to believe that you are spot on, with what you say.
I just bought a book on Paper Patching and long distance shooting. I believe if I shoot a paper patched Postell style bullet-seated out to the lands in my single shots, my accuracy will improve even more. Again this puts your comments spot on. We will see. Tom.