Having done this conversion, these are the problems, if you use a gtir manifold it flows better by its design, it is more centralised where the pipe comes out into the turbo so no grinding of the block, if you do it this way, your oil return line does not line up any more so a mod is needed, also you water cooling and oil feed from the block dont line up any more either i suggest a full set of braided from Midnight Mods on NS.com, your intake pipe is different as well, the O2 sensor may foul your airconditioning pipe if you have ac, the dump pipe you just use your current t25 it is the same but with the manifold sitting 1.5 inches more towards the front of the engine or no1 cylinder your exhaust may need a bit of adjustment as well. thats about it but yeah big gains to be had, the t28 from the gtir has a .86 rear housing as compared to the t25which has a .64 means you get more lag but will have more in the top end of the rev range, but wont cut in till 3600 rpm. i hope this helps, dont forget you will need to remap and s15 injectors.