If you do not have a pressure gauge, you should consider getting one. I would estimate that you have the 3 psi (due to water head above the diffuser (7foot x 0.433), but you also have to consider that the diffuser may add and additional 1psi of back pressure, plus the pressure loss in the 150' of linie (maybe another 1/2psi) This yields about 4 to 5 psi. According to your pump curve, that put your system at a 1.5CFM air supply. A pressure gauge at the pump would tell you for sure without the assumptions, but 4 to 5 psi will be a very close estimate.
If you do not have a lift rate for your diffuser, the rest of my rambling will get fuzzy and may not apply to your system. AND, the lift rate chart for the Vertex brand (the only lift rate chart that I am aware of) gets some doubt from some of the more experienced members here. At any rate here is the lift rate chart for Vertex. I will assume your diffuser is like the "XL1"...
Your system at 1.5 CFM is 0.5 CFM more than this chart was created with so your lift will a bit more than the following calculation (IF the "XL1" equivalence assumption is true).
At a diffuser depth of 7 feet, the chart indicates that you get 1200 gallons per minute lift...that yields (1200gpmx60{min/hr}x24{hours/day}) 1,728,000 gallons a day. Your pond is likely in the range of 1-1/2 to 1-3/4 million gallons total. So, you are making the One Turnover per Day goal. I'd say that you are doing fine with what you have IF your diffuser is near the performance of the Vertex XL1 as indicated on their lift rate chart and my assumptions for other pressure losses are close. Your diffuser should be located in the basic center of the deepest part to maximize the turnovers.
It's impossible to say that upgrading the diffuser will improve the system without empirical data on the alternate brands which seems to be non-existent. Be careful, some diffusers add more back pressure than others which would lessen your effectiveness.
All in all, your systems appears to be just big enough to meet the minimal one turn-over per day. Without finding the best/better diffuser, the next step would be to get a bigger pump that could supply more CFM/Pressure and add two diffusers. At this point, your 1/2" lines are on the small side. The downfall would be that you new pump would have to work harder than necessary, but you could probably double your turnovers.
