I have the EOS speed booster. The EOS to m42 adapter (random purchase), does allow infinity focus with no trouble. C-Curtagon 35/2.8, Biotar75/1.5, and such (not sure what else I have or tried).
PS: Checked again and it is spot on with no issues. The adapter is eBay# 320951898301. I think I have bought the other adapters (EOS to NikonF, OlympusOM, etc) from the same seller. They work fine for me.