s15 turbo should be round 280 @ 15-16psi, that turbo should hold 18psi no problem, and if you had a boost leak unless it was very very small, as soon as you boost it'd start bucking/coughing/acting a fool so i don't think it's that. still double check your coupler clamps and replace any dry rotting/cracked couplers. if your wastegate actuator arm is adjustable try taking a little slack out of it also. if you're running a manual boost controller then it may be actually set@ 16psi but is just spiking to 20. manual bc + internal wastegate = spike.

best bang for the buck = old school GReddy Profec. it's just like a mbc as far as adjustability but it holds true /w no spike the whole way across, plus you can set the gain higher to have the turbo come in sooner. i would definately not recommend assuming it's set to 16 but spiking at 20, so in compensating setting the base to 20 and having it spike to 22-24. it's cheaper to get a good boost controller than another motor.

what injectors? engine management?