I just completed the bulk of this project (minus the rear wishbone stone guards).
I've kept gas mileage logs for the first 30K miles on the car. I have a pretty standard drive /commute, and should know within a tank or 3 if my gas mileage is improving. As for road noise, I'll let everyone know what I think in a couple days.
To follow up, I would say that as for road noise, there is only a mild reduction. Actually, that is what I remember thinking in the days right after install, since at this point I no longer really have an A/B comparison environment, or accurate memory of how quiet it used to be... I guess it got a little quieter, but I wouldn't do these mods for that reason alone since the amount of sound deadening is only faint to mild. But that is just my opinion. It may be perceived to be quieter in the back seat, but from where I sit in the driver seat it is minimal. What I notice most prominently is tire growl noise, so it may be that wind/buffeting noise is reduced, but I am focused on the tire noise to not appreciate the sound reduction caused by less drag.
As for gas mileage, I was using the fuelly app to track mileage, fuel fill up costs, and fuel economy. Since doing the modification I have filled the tank 5 times, and wanted to report on my findings.
Jul 7th - 32.6 mpg (340 miles covered)
Jul 12th - 31.6 mpg (368 miles covered)
Jul 23rd - 33.4 mpg (383 miles covered)
July 30th - 33.0 mpg (383 miles covered)
August 7th - 32.8 mpg (394 miles covered)
----- Modification ----- August 10
August 12th - 34.4 mpg (334 miles covered)
August 19th - 35.5 mpg (383 miles covered)
August 26th - 36.1 mpg (435 miles covered)
September 3rd - 35.2 mpg (394 miles covered)
Sept 9th - 37.9 mpg (425 miles covered)
The last entry included a 240-mile road-trip that was outside of my normal work commute, but was all highway miles with minimal traffic. My personal all time best economy in this car was over 482 miles where I achieved 40.9 mpg (before the modification), and I hope to best this top score in the future.
Nevertheless, the average consumption across 1868 miles before the modification was 32.68 mpg. The average consumption across 1971 miles after the modification was 35.82 mpg. So, I am confident in saying that this mod DOES improve fuel economy, by almost 10% ! I haven't done the math to figure out how long it will take to pay for the materials or my time to install it... but it's nice to know I'm getting better fuel economy, and therefore should be able to drive further on a tank before needing to fill up again.