Just my .02
If you're dealing with a stock-ish engine, and a big 'ole aftermarket pump capable of supporting 600hp, you very likely need to use PWM.
A stock-ish engine will never ever come close to needing what the pump can supply. Pump running 100% all time is likely a recipe for early pump failure.
On the 5/16" line deal, many FI vehicles out there came with them/run them without issue. However if using the big 'ole aftermarket pump, it is probably a good idea to increase to 3/8" especially if they recommend it.
On mine using pumps matched more to engine needs, I've had zero issues with either of my TBI wags. One is 3/8" and in-tank pump, Other is 5/16" external in-line pump.
But, they are OE type pumps capable of supporting stock-ish engines or mildly modified engines. Both setups have many (tens of thousands), trouble free miles on them.
If I was to install say a new FiTech system, I'd upgrade line size, but I would run an OE style stocker pump in the tank.
If FiTech is saying you can get away with dropping it to 60% that tells you a lot about how much it is overkill for the system, and just not needed until you increase HP significantly.
Just my opinion though