Personally, I'd focus on the front control arms and forward. Make as smooth a radius as possible from the lower edge of the front clip to the bottom of the OEM airdam. From there, attach an undertray at least as far back as the control arms...but focus on making them smooth.
I think your biggest gain will be to fabricate some front wheel splitters and boat-tails...either out of reinforced coroplast, fiberglass, or some other stiff/cheap material. Radius the front edge and have it expand to where it ends
wider than the wheel. In the future, a key addition will be front wheel skirts so you might as well set up for that. The same idea holds for the boat tails. Have them start wider then the wheel (but with radiused edges) and taper back at a 7-10 degree angle.
In the future, the next step would be to add a splitter/boat-tail to the rear wheels and extend the undertray back to the bumper. I'd also think about running full length side-skirts at this time.
Easiest mods at the moment would be to lose the side mirror, seal all gaps, and use a lighter weight oil w/ oil analyses (pretty ballsy, though). Welcome to the land of diminishing returns...
- LostCause