I'm pretty sure that 911's have somewhere around like 450 horsepower.. so I can't imagine you keeping up all that well. I don't know that you'll be totally embarrassed but if they're gonna be going WOT the whole time they're out there you will probably find yourself getting left behind.
Hmm.. just googled a bit.. looks like 911's run 12's stock, and GT3's will be running low 11's..
Even with a bigger turbo you probably won't be keeping up all that well. You have to remember that those cars cost about 120,000 new... 4 times what your STI was. I would go and have a good time, don't worry about the rich people having a faster car