I can't speak for the FASS but from what I've read, the AirDog is a good set up. I put it on my completly stock Ex for a couple of reasons, none of them being performance related. Of all the fluids to add an auxilary filter to, fuel seems like it could do the most damage if you get a bad batch. Plus you fill up more with fuel then oil & coolant so the possibilities of getting bad fuel is greater than all the other fluids. After installing the AirDog, the truck is alot more quiet than before. I wired the relay trigger to the stock fuel pump so when it's on, the lift pump is on as well. Here were my reasons for adding this to my truck.
1. Water Seperation
2. Extra filtration
3. Air Seperation (didn't know this was a problem until I did some reading)
4. The stock fuel pump is 12 feet from the tank & I figured the low pressure lift pump of the AirDog could take some of the strain off the stock pump.