Congrats on the bubbles!
9 psi does sound a bit high, but, according to the pump curve, you are putting about 3 CFM to the diffusers which is a good place to be as far as air volume is concerned. You do have a fair amount of line lengths, but that should not account for much more than 1 psi of pressure loss. Add the 4 psi for the depth of the deepest diffuser and 1 or 2 psi for the resistance at the diffusers (what Bill was talking about) and you should be seeing about 7 psi total. 9 psi is not far from that so you are close to my rough estimates. If I were to suspect something, I would look closely at the valves (or system balance...see below). More times than not, a 1/2" valve, for example, will NOT have a 1/2" hole through it which constricts air flow at the valve (the valve hole will be much smaller). There could be your added pressure loss depending on your valve manifold sizing. I used 3/4" steel plumbing at my manifold, including the valves and such, then ran smaller 1/2" PEX line to the water and then 5/8" sinker line to the diffusers. This helped eliminate any tight constrictions at the valves. The 1/2" PEX was a bit of a constriction, technically, but it is what I had available and so it is.
All and all, you are in a good place. If you can get a psi or two regained, your pump will be working less and you will see a bit more air flow.
This could be of help too...(system balance)
I just reset all three of my diffusers (powered by the Gast 0523) by opening all three wide open. The pressure at the pump was at 5 psi (4 psi for my 8 foot deep diffuser and about 1 psi for diffuser and line back-pressure). BUT, as I choked back the more shallow diffusers to increase the boil at the deepest one, the gauge gradually increased to about 7 psi. I don't have near the line lengths that you do, but our systems are in the same ballpark. I can easily get the gauge to read 10 psi, but then the shallow diffusers are weak and the deep one is maxing out. You only have 2 diffusers which, when compared to 3 diffusers acts like a restriction or , better put, acts like a lack of outputs. I'm not saying you have to have 3 diffusers, but a third one would likely lessen your back-pressure.
Once again, your in a good place, just a bit close to the edge. The pump curve suggests that 10 psi is the max under normal operations.
