I'd start with maybe Vishnu. I hear he's begun playing with some pretty beefy turbos lately, could probably get you over 500 hp without breaking (much) stuff. However, an 11 second car requires one hell of a lot of horsepower, and I don't think it's reasonable to ask for that much boost below, say, 4500 RPM. That's not very streetable to me. Not to mention that you'll need a better tranny. The stock is easy to break when you start cracking 300 hp, unless you are driving specifically to protect it, and that's not the same as driving to go as fast as your engine will take you of course.

Anyways, I'd talk to the shops and companies that have lots of overall turbocharging experience, since what you're asking for isn't really something that you can get off-the-shelf. Even a dedicated Subaru tuner would basically be building a one-off kit to reach the power you want. No 500 horsepower group buys. So, try talking to live people at HKS, AVO, Cobb, etc. They can all point you in the right direction.

 

Posting here about getting a new turbo is fine, but it is possible you'll get better answers in the factory turbo forum.

 

No biggie. I jump to that conclusion when I see WRX in this forum too... I didn't really notice he was in the right place til the second time I read it.