[scheides]

A) if you had a leaky BOV before, you would run less boost. Now you have a decent one on and it holds boost better, so you are seeing more.
B) Search for info on recirculating BOV's. Your MAS reads the amount of air going into the engine. If you open up any part of your intake plumbing after that, the engine is getting less air than it thinks it is getting.
C) read more closely here, there may be something you're missing; can you adjust your wastegate actuator arm? Are all your hoses feeding positive pressure to your wastegate actuator tight and not leaking?
D) do you have a boost leak anywhere?

IMHO get the turbine housing ported, that will help solve the problem. Consider a different o2 housing, such as the LIPP ones that LSE sells.