I would recommend a 245x50x16 on front, since that's the max you can go with minimal to no rubbing at full left-right steering.
I believe 255x50x16 would be the max width on the 8" GTA wheels to keep a straight sidewall. Any larger will probably look rounded like a drag radial. A 255 rear will be appx 0.4" taller than the 245 which is a nice rake look plus a little better mph thru the traps.
Spend some time researching many different options on
http://www.tirerack.com/tires/SearchTires.jsp in particular the max performance and ultra high performance tires.
I've been debating and debating going to a Nitto drag radial vs. sticking with an ultra high performance tire. I like a nice looking car, so I don't want the rubber and rocks thrown all over the wheel wells and quarter panels. I'm leaning to getting the 255x50x16 Firestone Firehawks on the rears to compliment my 245 fronts. A little more meat and height.
Here's the list of 255's to consider:
Michelin Pilot Sport $267 H 26.0 W 10.2
Firestone Firehawk SZ50 EP $140 H 26.1 W 10.4
Dunlop SP Sport 8000 $129 H 26.1 W 10.0
Sumitomo HTR Z $91 H 26.2 W 10.2
Kumho ECSTA Supra 712 $87 H 26.1 W 10.4
I think the Michelin's are too pricey without any better performance than the mid-priced tires; the Sumitomo and Kumho supposedly perform very well at a great price.
Regardless, with your power you are going to blow away the tires on the street at will. Same problem you had with the Nittos. And you'll have to live with 1.9-2.2 60' times on the track. But WOW your high speed cornering will put a big smile on your face.