For 550 bhp + a T4 isn't going to do it.
Nor is a GT30 - even on some of the larger exhaust housings... AFAIK a GT30 is an ideal 450-530 ish range turbo.
Sounds like a GT35, or one of those fancy Turbos (Turbonetics?) Stavros posted about recently - sure he'll be on a here to supply a proper techy answer to this question!
Si