I have ran 4, 6, 8, 10, and 12.5 flow rates. I know I use more water at 12.5 but nowhere near double what I did at 6.
I have been doing a restaurant for a very long time and when I started doing it I was running 4 flow and always had more than enough with a 75 gal fresh water tank.
When I changed to 6 flow I just barely made it but always finished with 5 gal or so in the tank.
Then I went to 8 flow and always had at least a 12' x 20' area left when I ran out.
I switched to 12.5 just recently (5 #2.5s) and now have a 100 gal fresh tank. the last time I cleaned the same restaurant I had more than 5 but less than 10 gal left i the tank.
I always run at 550 to 600 psi.
Just thought this might answer some of the water cunsumption / flow questions.