yes, any parts that fit the GT (or the outback) will fit the legacy of the same year, 95 - 09.
Please be aware that the 2.5 GT has a different engine - turbocharged - that utilizes a different exhaust, ECU (computer), and other parts. It's a DOHC vs. the SOHC NA engine, too.
So, many of the non-engine components will swap, but not the engine stuff.
If you want more power, your best bet is to find a low mileage GT ('05-'11 (?)) - ensure you get a compression/leak down test!! - then you'll have access to more power options if the stock engine isn't enough.
The power upgrade options on non-turbo Subarus are generally not worth the modest gains you'll get for your $$.
Go spend some time on the LegacyGT.com site or http://www.sl-i.net/, too. They'll have more performance ideas.
Edited by wtdash, 12 May 2013 - 10:23 AM.