I looked at AD when they & FASS first came out they were really high, for 90+% of 6.5ers out there the h/o fom Heath is sufficient, I ran mine that way for about 3 years, I was just looking for that extra when towing big, I hooked up with folks @ Hoesli Diesel in Evansville Ind. they sold me a nearly new Walbro for $35 that was tried on a Cummins but didn't do what they wanted, and am not disappointed in it. FWIW FTB isn't mandantory at all, just an enhancement I cooked up, but it works, Heath's LSR ran FTB fittings I sent for another test I wanted him to run for me; & Todd convinced him to run the fittings & Air Dog at Bonneville
AD is vast improvement over Walbro to be sure, possibly overkill, as the DS-4/DB-2 will only take on & use X amount of fuel & rest is returned to tank, if you have to "dial down" the delivery of the AD, possibly too much pump for the 6.5.
My preference is to match the pump to the demand rather than reduce the delivery of a bigger pump, sort of like a turbo being sized to work in the engines optimal "power band" rather than have too much or too little of a turbo.
But that is me, do what suits your needs, I did maintain my Heath h/o plumbed in parallel with the Walbro so if the Walbro died, or I need even more flow (have not had to run that way yet) then I can switch on backup pump or both pumps.