Upgrade the headers, then tune it - this will really open up the top end. After that the laws of diminishing return kicks in, so isn't worth spending your money upgrading much else IMO
I'm about to purchase a set of these for mine:
http://raptorsc.com.au/subaru/subaru-libertylegacy-4-gen-ez30-header/
I've been looking at getting the Raptor headers for mine, they have the only aftermarket headers I've seen so far: http://www.raptorsc.com.au/subaru6.html