Open dump will help gainning a few hp i can't say how much but I did some research and they the all indicated a gain versus no dump. If you don't mine the sound than you should be fine. My open dump open at 4600rpm so normal driving I never get to hear it.

 

It doesn't necessarily have to be an open wastegate dump, recirculated is fine too. I'm just referring to an aftermarket o2 housing in general. I would say 15hp and 200rpm quicker spool is possible. I would also look into why your boost is tapering so much towards red line too. Could be that your wastegate preload needs adjusted, duty cycle is not optimized in the tune or the spring is too weak at your boost level.