Since the thing is so new, it's hard to find anything for it. All of my guesses are just from what how the WRX has been tuned. ECU reflash, bigger up-pipe or down-pipe, turbo back (if emission free, or you don't mind being illegal) exhaust may help, maybe a cat-back.... hard to say on that one though.
In many of the Solo IIs we've gone to, the STi has some serious (for a high-performance sedan) body roll. Stiffer suspension, I imagine, would help this. I read that the 2004 JDM STi has a little less horsepower (although, who can trust their horsepower ratings

), but a much better suspension. Could always try that if you had a good deal of money to blow. A rear sway bar would probably eliminate any understeer found. High performance tires.
Also, a lot of the stuff could just be by preference. For instance, my 2.5RS had no badging on the rear, along with the OEM sedan spoiler, powder coated wheels, and huge coffee can exhaust. I'm replacing all of this. Getting all of the badging and the spoiler from the factory. Keeping those wheels so I can be in a stock Solo II class but putting used WRX wheels (finding some used 2.5RS wheels is just plain annoying) for everyday driving. Plus a new, smaller, but still performance exhaust. But ya' see what I mean? You could rather make the STi feel/look how you want it to.
If you want something more than 300 horsepower and 300 ft/lbs of torque maybe you should look elsewhere. I imagine tuning it would be very expensive and take a lot of time and know-how. But I'd be willing to turn down many other more poweful vehicles in exchange for this one, just because I love Subarus. I'm all in favor of tuning it, and spending wads of cash on it, but I'd wait until I knew how to handle the already massive power it has. This would give time for companies to work the STi and get performance parts out for it.
A lot of this is just speculation. Things I'm firing out of that bad, bad, orifice.
PS. One thing I think I would have to have (depending on the stock unit)... big, loud, audible BOV.
