First of all, let me give you some background on the reason(s) I purchased my Ralliart. I live in Hawaii, and while it's fun to jam around the island late at night and pray that cops don't see me flash by in all my Wicked White glory, there's no where here for me to drive fast enough to justify spending an extra $10,000 on the Evo. Don't get me wrong, I seriously looked at the Evo because it's been my dream car since before that silly Fast and the Furious series even started. But I couldn't justify the extra expense when I could throw that money into some extra goodies, like upgraded seats, sound system, and quality window tint.
Basically what it comes down to is that Mitsubishi has the Ralliart positioned right where they want it; in between the GTS and the Evo. That said, it's hard for aftermarket manufacturers to justify mass producing performance parts for the Ralliart when those sort of upgrades (i.e. turbo kit, [insert your favorite aftermarket part here]) would essentially create an Evo-lite. Furthermore, the Ralliart's suspension, more so than the transmission, isn't really built to handle huge power gains. The stock suspension setup won't allow for the wider track that the Evo uses, so you would need to perform relatively major modifications to upgrade to Evo-standard suspension.
While the suspension, in my opinion, is the true barrier to the Ralliart becoming an Evo, let's not forget the transmission. I love the SST because I do a lot of city driving, and let's be honest...it's really smart. The reason it's smart however, is because it's run by a computer, a computer that's programmed to shift at different points based on a combination of speed, rpms, assumed power numbers, etc. Let's say that you decide to go buckwild and throw on a huge turbo upgrade with a larger intercooler and larger piping. The internals and injectors will probably be ok because they're essentially EVO parts. That's about the extent of the good news, however. Not only will your ECU have a seizure and spike itself in the crotch, but also the suspension isn't burly enough to let you put that much power to the ground, and while the transmission is similar to that of the EVO MR, it's not meant to take that much torque and power. When you look at all the EVO X builds that manufacturers are putting together today, most of them are on the GSR, not just because it's cheaper, but because the SST transmission COULD pose problems in terms of limiting usable power.
To answer your question, thirty minutes later
there are a couple/few reputable companies that have legitimate tuning parts out for the Ralliart. Basically, you're looking at exhaust, ECU, and intake.
Buschur Racing offers a larger aftermarket intercooler, along with stainless lower and upper IC piping. They also offer a mean cat-back (which is on it's way to me as we speak!) that bolts right on to the Ralliart.
AMS is another tuning house that typically specializes in Evos and STIs, and offers a similar lineup to that of Buschur, exhaust and intercooler, but also adds a ridiculous ECU flash that supposedly bumps up your hp and torque by 50 and 50 respectively. I have yet to hear from someone who had it installed, as you have to physically remove your ECU and send it to AMS for them to flash, but their reputation is flawless and I wouldn't doubt their statistics one bit.
Other than those two companies, I would recommend steering clear of bargain ebay specials and companies that don't specialize in Mitsubishi tuning. If you're looking for aftermarket parts that don't actually have a functional purpose, well, you're on your own
http://www.buschurracing.com
http://www.amsperformance.com