Removing the underhood cover will result in a definite lessening of rodent invasion. They love that cover material - particularly the fiber mat under the black outer layer. They think it is the best thing ever for nesting material. Particularly if one has been sitting for a few months over the winter, you are likely to find a large wad of that stuff around and on top of the cabin air filter. That seems to be their favorite nesting spot.
Hopefully that isn't as bad a problem for you, but up here it is a definite factor.
I doubt any change in thermal efficiency will be had in a statistically significant and measurable way.
The engine cover is supposed to help with noise. I haven't found the loss of mine to make any difference. And after a few years, the urethane foam that they originally came with degrades into grey dust and just makes a mess all over everything under the hood anyway, so by now your '02 won't have any foam to absorb sound, heat, or anything else. So losing the cover is really an aesthetic choice.
I removed the engine cover a long time ago. It was always in the way if I ever popped to hood to see if there is anything I need to worry about, and certainly cost extra time if I was going to do any maintenance work, so it was easier to just leave it off. I am sure I have one or two around here someplace. I'll just toss them sometime soon. Need to get rid of extra junk since I'm moving.
There are other items that can be changed to increase fuel efficiency. It seems to me, however, that the best one is the reduction of excessive turbocharger boost. You'll need to search for that one, but a tune is the best way to correct that. That should get you started.
Cheers,
PH
