Quick update. Went to AP performance and it's the best thing I ever did.
He discovered that the engine was knocking on the standard map using our 95 octane petrol. So he spent a few hours mapping it and getting it right, and it's like a different car after him. Peak power is now 323.1hp at 5700rpm and peak torque is 375ftLbs at 3500rpm.
This is on a completely stock 2.5 sti PPP. (Although he reckoned it could be de-catted as the catalyst monitoring had been turned off, so I must check this out). We went for a drive too and he showed me that there's no knocking on the data logs. It drives so well now, can't recommend him enough.