Nick on here is running around 360 on the 'stock' uk sti engine & as it's forged with standard i/c I can't remember what other mods it has, he was looking to get to 400 but requires some large amounts of cash throwing at it. Fmic & bigger turbo plus map for starters, fonzey has a build thread here and is on 400 bhp may be worth having a read through to get some idea's
The cat will be restricting the flow a little, I think the prodrive system has the sports cat so it will be less restrictive than standard,
Maybe worth getting it on the dyno to see what's going on, as if you went remap you could push it to safe levels of fueling and upping the boost, plus you could always go for switchable maps, low boost fuel save cruse map and an all out attack full mashings?