with out trying to sound rude either do a pile of research or just call up race-tech/tuning developments tell them How much power you want and give them your bank card because you seem clueless. A gt28 is just stupid on a stock engine. your going on about will 1.5mm oversized pistons solving the problem but the size of the pistons has nothing todo with what turbo your running. For a gt28 you really want a fully forged engine not bigger pistons so your realistically talking 2k for engine then fitting it and misc costs for running in and what not then a mappable ecu and mapping. So your gonna be 3k down before you know it. Take sometime browse all the tech sections and work out what you want and your budget. This whole stage 1,2,3 stuff is just daft when it comes to actualy explaining what's been done or your trying todo. Because everyone man.dog and tuning company had different ideas on mods make what stages.