Now that you mention it, a double head diffuser, like the vertex XL2S, could give you some improvement. Vertex states that their lift chart was developed with the "Air Flow = 1.0 CFM (cubic feet per minute) per disk". With that said, they claim that a double head diffuser actually doubles the lift. What we do not know is what happens if you double the CFM for one diffuser. My suspicion is that air flow beyond the 1 CFM per disk actually swells the membrane so that the slits open up further causing the bubbles to get larger which is thought to lessen the lift results. In other words... the increase in lift in not linear to the increase in CFM when considering just one diffuser. IF that's the case, adding a double head diffuser, even though they will not be operating at 1 CFM each, should give you some more lift. This is a good assumption given that the back-pressures are the same or better when comparing the old diffuser to the new. You have found yourself on that fine line that could go either way and you would not know if the modifications made an improvement without a lot of bench testing.
I would physically look at your boil and compare it to some videos of other adequate systems and see if they look the same. Not very scientific, but at least it will let you know if the violence of the boil is in the ballpark. If so, I'd say leave well enough alone until you want to take it to the next level.
