I agree with You, I dont have viltrox , i modified Sony NEX booster mount to go with my M in 2014 but i do agree you can get corner softness with some lenses but some work OK, i have 35-70mm zoom that works fine , there is some shift of focus plane that looks like softness, in reality when you look from the top then focus plane is single straight line, but with speedbooster it shifts with some lenses and becames " )" so when you hit infinity then middle is sharp but corners are actually over infinity thats why theyre soft, this is more and less visible depending on the lens and how its built, it just needs to match kinda OK with speedbooster lenses.
What i mean by less noise on M is that when i have f2.8 lens and i have to shoot at ISO 1600 on 5D then on M with the same lens and speedboster i can shoot at ISO800 and this gives me less noise.Also M with its cropmodes and speedboster eliminates soft corners issue , i believe canon 5D doesnt have as many cropmodes available ( 5x5 and 3x3)but i might be wrong.