Depends on your budget.
1. If you have the $$$ right now - go straight to a GT-T, then spend on the mods. Less hassles.
2. If your budget is not that much and you want to keep the car - I wouldn't bother spending much more on "power up mods", just on supporting mods to make the car more reliable when you drive it hard. Oil and trans coolers. Premium oils and fluids. Also focus on handling if you can stretch your budget a little.
3. de+t setup - Would only be in your favour if you have spent lots of money on the car already or if you just want something different. Reliability should not be an issue as I imagine your are going for something conservative, but be prepared to spend more in comparison to a straight GT-T.
Your current setup is not too bad - personally I'd get the trans done, add the oil and trans coolers - should be good.