Spent a day at Thunderhill last week, in my first-ever track day in a car. The event was put on by the Golden Gate Lotus Club, which was a fairly relaxed group. For those who have never seen (or heard of) Thunderhill, here is a satellite view of the track (Google apparently doesn't have hi-res imagery of Willows area).
There was plenty of track time available -- drivers were split into a "Fast" group and a "Faster" group; the "Fast" group was restricted to passing mostly on the three straightaways, whereas the "Faster" group was limited to drivers with multiple days of track experience, and had unrestricted passing. Sessions ran for half an hour and alternated between the two groups, which was pretty nice, since you could just glance at your watch and figure out how long until your next session was going out.
Getting out on the track was a blast. I took it pretty easy for the first couple sessions, especially after the driver's meeting comments about not spinning out in the first session, then started to push the car harder. The STi proved to be a very easy car to drive fast -- it has a basic tendency to push, even with the center diff unlocked, and towards the end of the day when I was sliding the car through turn 2 it transitioned cleanly from hard cornering into a gentle four-wheel slide.
I learned an incredible amount as the day went by, and I was going much faster by the last session than I was at the beginning of the day. My progress was marked by my gas consumption -- I burned two full tanks of gas at the track, going through an indicated half-tank of gas alone in my last session (although I must point out that it was the first half tank, which is smaller than the second half).
Tire wear was less than expected, but I think the stock tires are now nearing the end of their useful life anyways. I'll probably have to start shopping around for a new set soon, although finances being what they are, I may just throw the winter tires back on and run on them for a while.
I didn't really feel the need for any more power out of the car after the day -- certainly I was full on the gas in many parts of the course, but I had to feather the throttle through at least half the course, so I would say I am tire-limited (and skill-limited) rather than power-limited. One modification I would like would be some sort of four-point or five-point harness. I saw one of the (eight or nine) Mitsubishi Evo drivers had a harness put in his car that was pretty low-impact. The other thing is that I want a video mount for my camera...