I use the Falken Azeni ST-115 245/40-18's these are about as good as you can get for perfomance tires that will help you get traction (very important w/400HP) and keep the grip in the wet. If you want Max performance in DRY conditions, step up to the RT-615's that is one step below R Compund race tires.
My personal opinions on good tires for the GTO are as follows:
STREET Performance (Dry and moderate rain)
1.) Falken Azeni ST-115
STREET Performance (Dry only - Better than above mentioned)
1.) Falken Azeni RT-615
2.) BFGoodrich g-Force T/A KD
3.) Kuhmo ECSTA MX
RACE DOT
Hoosier R3S04
With the weight and HP of the GTO, these are my recommendations. My suspension is tuned, I have ZERO wheel hop and my oversteer is dialed in perfectly, so my tires tend to wear a little more rapidly than most.
There are many other good tires out there I did not mention, these are just my preferences.
GOOD LUCK!!!!