If you run 800 psi in fresh water you will definitely damage your membrane. You want to limit the product water flow to that rated for the membrane in terms of LPH or GPH, definitely not the 800 +/- PSI for ocean water.
Your sentence about "keep it a little lower and warning that it will get up to "max" more quickly" is telling - "max" is the maximum flow for the membrane, and that is reached at very low pressure in fresh water. The words "keep it a little lower" are hugely understated, as 200-300 psi is typically the fresh water range. Actually, household RO systems operate without any pump at all, just the water service pressure in the range of 35 psi or so.
SN68 Sangaris, Tropic Isle Harbor, FL