Alicia,
With the turbo models, there can be many reasons for poor acceleration. I may be able to provide some things to look into.

First of all, have your timing belt checked. I can't stress that enough because if that goes bad while you're driving, you might as well look for a new car. The 2.0L DOHC engines Mitsubishi engineered are known as “interference” engines. Sound bad…. it sure is! One slip of the belt and crash, bang, crunch – your valves are destroyed and piston heads could be ruined. That's a $2500+ repair that I'd hate to see happen. The code computer may not always be able to detect when the timing is off.

If the timing is fine (let's hope), you may want to check your turbocharger itself. If you're feeling lack of acceleration at high RPMs, you may have a leak in your turbo intake hose, compression section, intercooler, blow-off valve, throttle body hose, or maybe even your throttle body. There are so many little vacuum hoses in the turbo engines, it only takes one leaky one to cause acceleration problems. If your car has cruise control (and I believe that's standard on a GS-T), engage it next time you're doing highway speeds. The cruise control relies on the vacuum system to be in working order for it to be engaged. If you cannot engage it, you may have a vacuum hose leak.

I hope these few things I suggested can lead to you finding the exact problem with your car. Good Luck!