The GPD of a Dow Filmtec membrane is determined at 50 psi and 77 degrees F with a softened tap water TDS of 250. These numbers were picked as a standard to be able to compare one membrane against another and are in no way the perfect conditions under which to operate. When conditions change, either up or down in temperature, pressure or total dissolved solids content, or in any combination of them, the GPD and the membranes rejection rate change accordingly. Some of us may have warmer water, or higher or lower pressure or in the case of Phoenix and the Southwest in general, higher TDS and hardness than the average user, so our output and efficiency changes. This is why I cringe when I see vendors promoting a RO or RO/DI as a plug and play or one size fits all with a fixed flow restrictor. Everyone's conditions are different so to get the best out of a RO/DI you need to fine tune the waste ratio and filter micron sizes etc.
Boosting is always good, even if you have 60-80 psi since it improves not only the GPD but also the rejection rate meaning DI lasts longer and works better, saving you money and landfill space from the spent resin. Remember, for every 2% you can improve that waste ratio or removal efficiency, like in 96% up to 98% or even higher, you double the life expectancy of your DI resin. In my case between going with a hand tested and guaranteed 99% rejection rate RO membrane and a booster pump at 95-100 psi my DI resin life went from 150 measured gallons per cartridge to over 1000 gallons per cartridge so the cost of the better boosted RO/DI system was easily justified. I dropped from 150 GPD at 65 psi un boosted with the dual membrane system to 130 GPD at 95-100 psi with the boosted system but my RO only TDS went from 13-15 with the old system to 2-3 TDS with the new system thus the longer DI life. I don't even need 130 GPD but it is nice to make wate rthat quickly and at 99.4% efficiency.