Get the Decat downpipe from JS Tuning, I'm soon to get one, I've ordered a KN panel filter although I'm pretty sure it won't make much difference, after that decide whether you want a FMIC( maybe 10-15bhp but is it worth the hassle)? I like the idea of a FMIC but don't think I'll ever bother. Once you have finished modding mechanically sell your tuning box and get it custom remapped + EGR delete. I personally think 200bhp is the most you want from this engine, to keep drivability, economy and reliability.
What year is your GT John?