The general consensus here is going to be that a "tuner" (especially a cheap one) isn't going to do anything, and if it does it certainly won't help with the longevity of your engine. A good old fashioned tune up is going to be the best way to get some performance and efficiency back out of your Pathfinder.

 

You having a VG33 there's a few things you can do to squeak a bit more power out of it (like 3.0 cams) if you'd really like, but more than anything that'll be for the fun of modifying it.

 

Basically, make sure it's in tip-top running condition and accept that realistically that's all you'll get.