WSIR and Button Willow are home tracks for me.
The difference between a mediocre time and a stunning time is all in turn 8 -9 and a bit in turn 1. Self preservation reflexs ( huevos) are the only thing keeping most from great times at this track. You have to work up to it slowly, 100 rpm at a time. Most think the cars will not hold.
8 is a no lift turn for the fastest times, drive right straight through the apex . It's a little bumpy down low like this but you have the straightest line. Then brake almost as if you were going to drive off the track into 9. Turn and dive into the apex at full throttle ( work's in the C5ZO6), there is a slight bump right there, just let the car settle and work up your speed here. The G's will pull you to the left at the entrance to the pit wall. If you just clip the dirt you hit it about perfect.
I have a C5 Z06 with a set of T1 bars, Nitto R2's and Long tubes. Nothing fancy and am able to Run 1:31 here pretty consistent. I have seen 28's-29's in caged T1 cars that are actually a bit heavier ( cage weight ). I like the Button Willow Track better. The speeds are slower with lots of drifting action in the Z. With the bus stop and not going all the way to star mazda the Z will run in the 1:57 - 1:58 range on R2's. I did an enduro last May and cut a 2:00:50 lap in a 5.0 A sedan car. Had several laps at 2:01.
I would think the GT would have an edge on the Z through 6-7, 8, 9 and on the straight. With decent sticky tires I would guess 1:28-1:29's with an experienced driver is no big deal here. Hoosiers should bring it into the 27 range.
I had no trouble going fast at WSIR in my AS 93 Cobra, the security of the cage, someone elses money.....makes you go fast a lot easier. When the C5 Zo6 was new I ran 1:38's on stock tires the first day and got to 36's day 2. It's just seat time that makes you comfortable with the car. More is better. If you have not been to a school try the track in Pahrump Nv, it's a Z06 school, Spring Mountain. The track is built to train. It's an outstanding course and far better than others I have been too in the past. Better to learn in someone elses car than your own IMO. Speak with Victor or Dave. They will most likely let you run some laps in your car at the end of the class.
The C5 Z is an amazing track car, much easier to drive fast than the Vipers. You will gain a real appreication for it after a few hots laps with Dave or Victor.
More power is seldom the issue on a road course. I instruct at a few clubs and seat time is the big factor. If you are going to mod for the track I suggest you start with Belts, Tires, figure out tire pressures for the track you are running. I see too many folks try to up horsepower, they get a little faster but only on the straights. A good driver in a 98 M3 with a set of Hoosiers will easily take a novice driver in a 360, FGT, Z06, GT3. I have pissed off lots of guys like this in a 5.0 Mustang.
The FGT was built for this action and should run at or near the Top of the class. Seat time, Seat time , Seat time:thumbsup