[Lemming]

Back when I did TTB and GTS3 with NASA (924S with 3L) I was the lowest HP, but lightest car. Great in many respects but got killed on long straights (where it is easy to pass), by higher HP cars with the same power to weight.

996 TB is a good idea, but finding a good used mechanical unit is not easy and new ones are over $1,000. Have you considered having your TB bored? Not sure if you will get the 10 HP that you need, but you will get some extra HP for those long straights at VIR.

You straight that you have full exhaust, do you mean stock?

You can tune the car and expect to get a few more HP as well. I would stay away from a preset tune and get one done with the car on a dyno for best results.