my car was also doing this, not going good, missing, cutting out, having a massive flat spot over 4,000rpm.
My mate at MSC PERFORMANCE found the best way to overcome this. My boost is set at 11psi or so. Afr's are around 11. And not revvs out to 7,000rpm no probs. Plugs are regapped aswell.
As a result a total of 209.5rwkw with standard turbo and ecu was achieved.
If you are around the melb/northern area. Pop into the workshop and see what he can do for you.