Wait a minute... with all those other mods how do you know your FASS gave you 5%? The Dynatrac kit alone should be good for 2 or 3% gain in fuel mileage.
I don't calculate partial-tank mileage because I've found it exceptionally unreliable. I once hand-calculated 44 MPG from a partial tank. I don't even pay much attention to the mileage per tankful, because I also find that highly unreliable (I don't have any faith in my own ability to consistently refill the tank to the same level, and even minor changes have a large impact on calculated fuel economy).
It should be obvious at this point that I'm pretty skeptical about the 5% gain from FASS, only because I have yet to see a single bit of data that supports the notion. I'm not calling anyone a liar - far from it. I think people getting 5% gain are genuinely reporting what they've found. Unfortunately, it's not uncommon for people to find precisely what they're looking for.
I have no idea whether the entrained-air removal system in the FASS actually works. Nor am I qualified in any way to pass any judgement whatsoever on the FASS, positive or negative. I have absolutely no data on FASS, and no experience whatsoever with the FASS system. I am, as it were, utterly ignorant of the FASS on a practical basis. I've never even laid eyes on a vehicle equipped with one, much less ever touched a FASS.
The only thing I can say with absolute certainty is that my LP fuel system had absolutely no effect whatsoever on fuel economy. Everything else, including my personal choice not to use a FASS, must be taken as absolute opinion.
So, to answer the question at the beginning of this thread, the best fuel system is the one you decide to purchase after exhaustive personal research.
-Ryan