After speaking to Simon at Sitech about turbos he said he had a car VERY similar spec to mine;
2.1 fully forged bottom end
Cosworth Managed
Big spec CVH head, Focus RS headgasket and Newman Cam
XR3 inlet
GRS Cooler

and it made 290BHP with a GT28RS Turbo.



I think 320 would be optimistic but not impossible.