i'd vote for an undertray to. it's a little more involving than an airdam, but it has so much advantages. even my first relative small one made out of a sheet of coroplast made such a difference in the way the car felt, it seemed more stable and quite. i once drove a day or two with my current dam removed the car felt wobbly and unstable... and while my car has a terible ammount of oversteer, the dam actually gives is the stability that allows my to toss it around a sharp corner or roundabout. it will still oversteer at some point but i've never found this to be a problem with in normal driving, even at high speeds
it also protects your engine from debris and dirt
what i most certainly recommend you do is to evaluate your mods.
you could do several things. first of all record your milage and work out if there's any change, but don't draw conclusions to soon... the best thing to do is do A-B-A testing over the same track, as weather and traffic all influence your results. if you have a fuelcomputer that tells you your milage instanly thay's even better.
you could also do some diy tuft testing especially on the vortex generators that should tell you one or two things
this aoutospeed article has some nice info on how to tuft test