when i first put my GTA's on my Regal, they had 255/60's on all 4 of them, and they cleared perfect in the rear. at the time, i had just one set- 2 fronts, 2 rears- with the fronts on the rear and the rears on the front with 5/16"' spacers.
everything cleared and fit perfect.
the car looked flat out evil with the 255's all around. the GNX flares are just something that ASC tacked onto the cars to make them stand out from the "normal" GN and really serve no purpose beyond looks.