hi, just wanted to know if the st205 water to air intercooler would be sufficient at the 350-400 whp range? Have read on some forums that it may be good enough for 350whp.

Good luck finding an answer! In all honesty you'd be better off getting a cheaper ebay water to Air IC and sit it above the air filter. It just depends if you want that OEM look.

Frozen Boost have a large range of W2A units that will cover your needs.
http://www.frozenboost.com/index.php?cPath=218&osCsid=942d269d88e24fdb4991ad993f46b885
Few AW11 guys have been running these with good success.

I would rather keep the st205 wta if it can withstand 350whp. I have already bought the parts needed for the st205 install. Previous power goal was under 300whp. But might as well go for more since the turbo will support it.

It's not the best, but IMO it'll do the job. Much better than a sidemount. Sounds like you're chasing shy of 300rwkw on a GT30 sized turbo, which isn't a ridiculous amount.
 
I've heard the ST205 heat exchanger is quite good & fits up front well. I considered that route long ago but there were none available so I just went the custom route.

Does anybody know the inlet/outlet sizes for the st205 w2a cooler core? Copper heater lines have leaked and corroded, but the plan was to use them to go up to the front of the car so needing to replace with hose

18mm ID/22mm OD inlet and outlet