To more succinctly answer your question: I have tuned hundreds and hundreds of Subarus, many at 500+whp. The answer to your question really depends on a few more simple questions:
(1) Are you looking for a 500whp dyno sheet?
(2) Are you actually looking to run 500whp while driving around town?
(3) Are you planning on doing track racing (open track, not drag racing)
(4) Are you planning on drag racing the car?
(5) Do you have a good source for C16 or Q16 fuel.
(6) Do you realize that 500whp is > 500 crankhp?
The answer to those questions will significantly help guide you. While an FP Red can make 500+whp, it has to be pushed very hard to do it. Take a look at this thread from more than 3 years ago: http://forums.nasioc.com/forums/show...onaugle+fp+red
If the answer to question 1 is yes, but 2 is no, the Red will work fine. If you really want to drive around with 500whp, you will need something larger. While a 35R can do it, you would still be pushing that turbo very hard, especially on stock heads and cams.
Consider the questions I posed above, and you will be able to get more help then you expected.