Just another thing for you. Before getting my car live mapped i was running 18psi on a T4, this was alot faster than a T34 car running 27 psi peak and 23 holding.

As chip has said you need to look at the bigger picture, it comes down to which turbo can flow the most air. The way you are looking at it would mean that you could run a T25 instead of a T4/GT30 and get 500bhp on a YB just by upping the boost to the same level as what they would need.