psi not right

i am trying to get my car to 18 psi and have a gillis installed.first try i hit about 22 psi and turned it down. then i was getting 18/19 at good rpm for awhile. now i am getting 18 but only at 5000 rpm. i did find an exhaust leak on my downpipe. could that be why it takes to long to boost? also when i get it replaced should i just get the 3inch even though i dont have the manifold ported yet? (87 5-spd dual 2 1/2 exhaust)

A leak after the turbine housing will not affect boost.

On a stock 87-88 with a gillis you ought to see full boost between 3300 and 3500. Possible problems are a boost leak somewhere, Wastegate problems, exhaust leak before the turbine, maybe turbo blades hitting housing.