I would ask, but I don't think you ever ran your car on 265 wide street tires did you?
I did, and I did not enjoy it. I didn't feel like I was ever able to make the car do anything. It just howled around the course being slow. I actually pax'ed decently in STX though... I guess .827 is friendly. Granted, it wasn't in the strongest of regions.
I genuinely have fun in STU. 1 extra inch of wheel and 20 extra mm of tire REALLY makes the car more fun. IDK. It doesn't make a lot of sense, the jumps in size aren't that huge. I actually have a better time with the 285 street tires than race tires. I guess it's a happy medium for me. Sure, I go faster on race tires...but you kind of just point the car where you want to go and it sticks, at least more so than it does with RS3's. At my skill level, I feel like I get closer to maximizing the potential of the car on street tires than I do on race tires.
The STAC has received A LOT of "we're pissed off about C5's" letters...so maybe we'll see some movement.
I think F stock is no good. Can you go fast? Yeah on A6's. See: the last several F Stock national champions. However, I like to modify, and modifying around a set of noodle-springs would be maddening. Kind of out of the question for me given what's already done to the car.
ESP? Science project. If you "max out" your car it will not be very streetable and you'll need to cut on it. I guess I could go lose to Mark Madarash all the time...that sounds like fun.
STU allows me to do fun mods, run with entertaining tires, and "arrive and drive." Here, I get to lose to 4WD turbo cars. Also, there are more people running STU than ESP around here by A LOT, and it's fun to compete.
Really- just pick who you want to lose to, and how much you want to spend on tires, and how much time you want to spend on your car at the event. The answers to those questions will class your car for you.