What are you measuring boost with? Riv'* don't have gauges. Whatever you're using, do you trust it?
Coupler, oil, intake, plugs, and fuel pressure have nothing to do with how much boost you'll get.
Belt slip, vacuum leaks, and boost leaks, as well as inaccurate gauges will all be possibilities.
Or another possiblity is that your PCM is limiting boost for another reason. Do you own a scantool? (I'm betting not yet from your previous posts and my PM'* from Slug).