It's about pressure, not so much volume. My sureflo for instance turned up to max is still not capable of giving me more volume than what comes out of my tap. So if I attach a garden hose to it in actual fact less volume comes out. However, if I try to stop the flow with my finger with the pump turned on, it's nearly impossible. My sump pump that I use to transfer water from my storage tank to my barrels on the other hand, delivers high volume and would soak you to the skin in a second, but if you put your hand over it you can stop the flow easily. It's low pressure.
Your RO requires high pressure but low volume. You won't get .85 litres/min out of the RO. Remember, it produces waste water as well. You'll be lucky to get 25 litesr/hour. So I would have thought that .85 liters/min at 70 psi, maintained at the input of the RO would have been more than enough to increase it's output by a reasonable margin.