underbelly pan would be negligible I think... the whole front end of the car is pretty much covered, and the rest is pretty well designed.
might as well go with a short-ram or cold air intake if youre gonna plan on modifying the setup at all, will probably net you better increase anyway, just my .02.
Once upon a time I had:
2012 SGM SEL Sedan Job 1 BD 6/2011
FSWERKS Cool-Flo w/ ITG Filter | FSWERKS Stealth Exhaust | 17x7.5 Axis Sport XMX Graphite wheels | Eibach Sportlines on Ti Handling Pkg bits