the best way would be to swap it...n/a modding wont get you very far, ive learned this, its one thing in an impreza...but its yet another in a legacy...its quite a bit heavier.
a swap always will allow for the most power just becuase you can mod after the swap. but swaps are not for everyone. a few aother ideas: turbo kit. AVO has a kit that will do 6psi for around 3.5k? i think. its kinda pricey, you could build your own for less, but you need to what your doing then. but they boast that 6psi on a 2.5l will put down 200+whp. NA mod. this is far more $/HP then boost. but if you dont want too much more power it might be the way to go. Exhuast, Headers, Intake, Em unit, some spicey cams, and you have just gained a considerable amount of power. of course you can always get more. TWE in canada has a NA all motor RS that is putting out 200+whp ~300hp at the crank. the thing is a monster and it is modded to an extreme no DD level. after thoughts. a turbo kit is almost as expensive as a swap. becuase of this it might be worth it to go the swap route. NA modding is pricey just becuase you need to spend a decent amount of money on parts that migt give a few barely feelable hp. IE exhuast is a couple hundered dollars and you might or might not be able to feel a power difference.
Just do like this legacy Sti.:rockwoot: http://forums.nasioc.com/forums/showthread.php?t=921576 or like Ben legacy H6. http://forums.nasioc.com/forums/showthread.php?t=748513
