I have a proper GT2871R mounted on a full equal length manifold with all the extras inc internals and made 360 BHP, could be more but it is a rich map to keep EGTS down. Yours certainly wont spool faster than mine. Power is strong from 3500 - 3600 in mine. GT2X elim was about the same, so yours can only be later i would have thought.
I reckon around 330 - 340 BHP.