As for the stability issue, as I stated before it might be a placebo effect, but what an effect.
I have a nice freeway here in San Diego where I take off underpanel and see if I notice a difference at speed. I'll do this when I get a chance.
As for the gas mileage, my last tank with me driving with A/C on for about 3 hours total and lending my Fit to my wife for a day netted me a 39 MPG over 9.3 gallons of gas (well that was how much the pump said). I usually cruise between 65 to 75 MPH with some DFCO thrown in during hills and such.
This week looks like I might actually break over 40.3 MPG (my best to date) with some limited use of A/C and no wife driving it like she stole it (she has an RSX Type-S for that). I thank the Laille/BeatRush underpanel....
(small dose of sarcasm)