I finally got my trailer, and because of that I am feeling like I could use every extra HP that I can get. I'm not expecting any miracles, but another 10 or 20 HP without spending a fortune would be nice, plus some things have other benefits too. I've been looking around at what sorts of things can be done in this area and have come up with a list of possible options, but I wanted to get some opinions on them. First, I'm wondering if I could mess up my powertrain warranty doing any of this, since more power = more stress on the drive train. I wouldn't think that some of these things are a problem, but I'm not sure about all of them. Anyway, here's what I'm thinking:
<$100:
High quality spark plugs
Switch to synthetic oil
High quality air filter (not oiled - I know, I know, you've run one for 8 gajillion miles with no trouble, but I'm not comfortable with it) (I'm not even sure they make any dry ones for a KK so maybe this is not an option anyway. Get on the ball AMSOIL!

)
>$100:
Air intake
Exhaust
The synthetic oil and air filter would also have the advantage of needing to be replaced less often so they would probably about pay for themselves, ultimately. The intake and exhaust, being more expensive, are probably more future ideas than immediate purchases, but I am wondering if they're something I would be able to install myself. I haven't done a lot of work on cars, but I do consider myself pretty mechanically inclined so if they're not too complex to install I think I could handle it.
Anything I listed not a good idea? Anything I'm missing? Suggestions on specific parts to use? Thanks.