As for the undertray it seems as though the amount of benefit is dependent upon how high your car is and how rough the underbody is. If your car sits low to the ground already then the undertray may not be that effective. However, the higher it sits and the rougher the underside then it may have more of an effect. That's what I gleaned from reading a VERY long thread on the subject. If I misinterpreted it then by all means let me (and everyone else) know.
PS When doing your aeromods try to do the ABA testing so you have before and after mod baselines for comparisons.
Thanks.