It hasn't been too bad with GRID, there is little in the way of any drifting (unless you choose specific cars and races), otherwise the physics aren't too bad.
Population numbers is the issue. GRID I know has 15+ players in CMP, PC2 has 4 people who own it in CMP (that I know of) and Assetto has less than that.
I guess if you can create an open tournament, I'm sure the CMP guys would join in.