When I was running the 12cm^2 factory Holset exhast housing, I was in the same boat. In an attempt to get enough flow from the wastegate I ended up removing the divider down to the wastegate hole, and replacing the wastegate flapper with a larger one / ported the crap out of the wastegate hole. It worked alright and would hold 10psi, creeping to 13psi at redline. Eventually I wanted a bit faster spool and changed exhaust housings and went external. I should have went external from the start. It controls the system pressure more accurately not to mention the time, money, and effort I spent trying to get the stock internally gated setup to work.