I had similar results, not on the dyno but I could feel the car fall flat at a certain point. Was advanced 4* through the whole map from FM's default, dropped it down 2* and the car woke up in higher RPM.

Aside from the well known wastegate issues, does anyone have any experience or knowledge of other related problems or Greddy's being maxed out at a certain boost level?

the compressor flow map is pretty anemic only 430 or so cfm max flow is not going to give you alot of HP the exhaust section also seems pretty darn restrictive i think with some porting there etc you could do a little better. I think some have switched the compressor wheels to a 16t or something similiar to get a few more cfm as well.

Nice numbers! At what pressure does the greddy turbo become in-efficient?