Unfortunately there aren't a whole lot of "bolt on" upgrades you can make to your NA 2.5.
Bigger (not by much) aftermarket exhaust can help and maybe a high flow air filter.
Switching up to a higher quality oil and filter combo can help with the "butt dyno" as well.
If you are willing to tear your engine down and get some machining work done, you can bump compression with a head shave and rework your head flow with a port and polish as well as getting a torquier cam.
But that's just traditional hot rodder stuff. You might be able to find someone to retune your fuel mapping for a decent price.
And if you start running out of fuel, you can upgrade to the bigger injectors found in the turbo cars.
Those are about all I can think of, maybe some one else can chime in?
PS: And the aftermarket ebay turbo kits are always an option, but you MUST retune after an install if you don't want to blow anything up.
Edited by Twitch de la Brat, 05 August 2014 - 10:53 AM.