GT6 has heavy steering already, perhaps it is your wheel or your settings. I use a Fanatec Elite and the FFB in GT is plenty strong, enough that I can't set the game higher than 5 without possibly straining a muscle while racing, even at 5 it hurts a bit when you hit something or get hit.
I do wish the GT would make use of the Vibration motors in the wheel though, would be nice to have a bit of vibration when going over a rumble or rough pavement