I've got a GT3 and it still has cogging. It's ranges from 'you don't notice it' to annoying depending on the car and track.
GTR cars on long sweeping corners are the worst for cogging.
I assumed you meant Frex when you typed Fanatec... While I don't own a Frex, I read over at RSC that the Frex does suffer from cogging. Also from the DIY section of RSC I've noted that finding a motor that doesn't suffer from cogging is very difficult. Last time I checked people were experimenting with Pancake motor which IIRC claim to virtual elimate cogging
How did you mod you Logitec gears?