.63 housing on what turbo mate?

If you mean a T34, that housing is more free flowing than the compressor side of the turbo anyway essentially, hence how little power difference there is between a .55 exhaust and a .63, so you still have a fair bit of headroom for more power, especially as obviously a lot of it will now be going down the wastegate anyway to prevent excess boost.

100bhp of gas on a T34 YB is a fabulous thing to drive but the trouble is the gas is expenive and its very addictive.