GTR suspension is at least twice as stiff as the M3. The GTR in "comfort" mode is much harsher than the M3 in Sport mode. Even if you have stiffer springs on the M3 the GTR is still much harsher.
GTR lip scrapes on the smallest of inclines
GTR is missing a lot of the nice touches of the M3 and BMW in general.
I have driven both extensively, I can probably answer any questions you have.
The GTR is the closest thing you will find to
this on the street unless you have Veyron money.