Well for lack of any decent measurement equipment i'm quite satisfied when i dont hear my system while sitting in my chair, when i can feel hot air exiting from the rad, and when i can get pretty decent OC from my hardware
So yeah 40°C is nothing if you cant push your CPU further it means you've hit a wall anyway.
FYI I have racked servers that run 24/24 between 55°C and 60°C... and they run fine !