The key to eliminating boost creep is ALL in porting the turbine housing. If you put a 50mm valve in there and dont do any porting, youre still screwed. If you keep the stock flapper, you can still get boost down to pump gas levels by proper porting. Its all in the vfaqs really. Another thing to consider is if you get a larger flapper, forget about running high boost efficiently. Bigger flappers blow open easily. Many people that think the boost dropping as rpm goes up is because they are outflow the turbo are actually see the WG blow open. I did the experiment, search this site for a thread named wg blowing open info for an experiment I did to confirm this
Hope that helps.