turbo stocker

Hello new guy with some questions, Im new to turbo's and here is my question, I have a 1988 thunderbird 2.3T with an IHI turbo, the class I race you must retain the original style/size turbo, I would just bolt on a t3/t4 hybrid if I were permitted but Im not, so if I were to have a custom turbo built with larger compressor wheels installed and the covers machined to accept them, what kind of performance should I expect? I realize the inlet is still going to be small but with the bigger wheels do you guys think the expense will be worth the gain? the one on my car now makes 18 lbs. of boost but stops making power above 16 lbs, I realize this turbo would cost $700 to $1000 range to have built but was wondering if its worth the expense? and if anyone here has done anything like this before and what were your gains? thanks in advance .............fred