[jrohner]

Increased exhaust backpressure is where the boost taper comes in mostly, but air seeping by the MBC does attribute to it as well. My 16G could only hold 22 psi, I gained a few psi by using a Holset WGA. If your wastegate spring is too light, you can also end up with the wastegate fluttering at certain RPMs; you'll see your boost gauge bounce around a lot.