Per Thomas' website for the current 2660 model, they run 4.6cfm at 0 psi down to 3.6cfm at 40 psi which is max continuous pressure rating.
That said, it is a positive displacement pump. It will generate the pressure required to deliver the massflow. Essentially, the line and diffuser losses. It will vary totally with what you hook on.
As max pressure occurs at the pump and that is what you want to protect, regulate there. From the photos, it looks like the two heads are manifolded together across the top of the unit. You could put the gauge on one output port and discharge from the other, pressures should be pretty much the same. At max flow, you would likely get a little more out of it if you discharge from both outlets and manifold together using larger than 1/4" tubing as you'd avoid losses through the manifold between heads and have lower velocity/loss than via single outlet.
