Quite a few people complain about drag racing with the STI 6MT due to the gearing. Also don't forget the effort/money required for rear diff massaging. It can be done with the stock WRX R160 rear diff and axles, but... Other than that, people seem to like them. Pricy.

Another option is a set from Gimmie Gears

Another option is a set from PAR, available from XX Tuning in CT.

And of course KAAZ and the V6 STI RA and V6 STI non-RA (same quality of gears, different ratios). I've heard nothing but good things about the RA ratios, with the exception of the inflated-RPM cruising in 5th for highway use. I'm not sure if they're the right bet if you're intending over ~350WHP though.

Then again, it all depends on your intended shifting.

If your intended goal is to bang through all 5 all the time and flat-foot shift, seems to me the reasonable answer is the TXS set.