I use the 6.8 lpm and it's not a massive difference, from 5.2 lpm
My pumps connected direct to van battery, Wen am on a job were I can leave engine running, you can see the difference then, the pump really ramps up the revs and the flow is much better.
I had the theory that a 10 lpm pump from vyair was the way forward but in reality the hose we use just isn't wide enough to carry the water, pump cuts out, kicks in, cuts out repeatedly and no pressure at brush head