I just put the FASS on my Volvo 780. I had it on my 1 ton Dodge cummins engine 2000 model year.
The Dodge had 100 hp injectors and a power puck with a new injector pump all at the same time. The cummins needed the extra fuel with the upgrade in power.
Doing it that way I was spec'd with the FASS B rated motor and pump rated at 90 gal per hour.
I was told by FASS that it would be enough for the 465 HP Volvo D12 I have.
The A stamped on the motor and pump is rated for 150 GPH, I'm not sure if this is making a difference on my truck or not. It seems to be ok though...I see the FASS this way, it is going to get the water and air out and it will filter down to whatever micron filter you choose. It will if installed properly keep cool fuel going into the head rail.
I had trouble finding filters at first but now I use the BT 372-10 and the BF 1212 and they are not hard to find. Both are the smallest micron I could find locally.
I too did not like nor was impressed with the customer service guys with FASS, In fact I'm waiting for a call back which I don't believe I will get unless I call back.