Welcome....I know those hills though I do live in town so don't have to deal with them regularly. I too was into performance and I agree that performance mods can help MPG if they are aimed at increasing engine efficiency and you drive for mileage. That is especially true if your normal driving terrain requires that you use your gas pedal for more than just a place to rest your right foot. I think that your belly pan probably only makes a measurable difference at highway speeds and I would pay attention during the winter to make sure that it doesn't ride up on deep snow and leave you with your wheels spinning. That happened to me once in my Corvette, I backed over a snow bank and the spare tire cover under the rear rode up onto the snow and took enough weight off my rear wheels that I couldn't move.
Good luck with your efforts.