Bill has shot you some great info (as Bill usually does!)
Regarding your 1/2 pex going 275' to a manifold by pond...
Some crude calculations using the following
Online Compressed Air Pipeline Pressure Drop Calculator...
https://www.engineeringtoolbox.com/pressure-drop-compressed-air-pipes-d_852.htmlUsing 4 CFM air flow (a decent estimate), 275 feet of line, and 10 psi, We get the following pressure losses for the noted line sizes...
1/2" ID PEX will steal close to 7psi from your pump,
5/8" takes over 2 psi,
3/4" wants less than one, and
1" requires less than 1/4 psi.
Keep in mind that once the line size is increased, the pressure drops and the air flow changes (increases), but the above numbers used in the calculator won't chane enough to ruin the example, just alter the results slightly.
So, you can see that jumping to 5/8" ID line should reduce your pump pressure by about 5 psi (leaving you pump runing at 7psi - still a little high for my liking) and going to the bigger 3/4" would leave your pump running around 6psi (not bad, but could be better). Going with 1" could reduce your pump back pressure to just over 5psi. The 3/4 to 1" jump may not buy you enough to warrant the added cost so those two are your choices. 3/4" will work well with 3 or 4 diffusers, but one inch would allow for future upgrades should you chose to push more air (bigger pump) through more than 4 diffusers.
The above mumbo jumbo relies alot on the remaining system's design dimensions. Your manifold pieces should, ideally, have internal dimensions the are 5/8" inner diameter (or close to that ID -1/2 would be OK). If you have fittings or valves at the pond that restrict the air flow much beyond the hose ID, you will see a back pressure increase from that restriction. You are already pushing air through a 5/8" hose...why try to cram it through a fitting with a lesser internal diameter.
Let's assume that you get the 3/4" feed line and your pump's backpressure lands at 5psi. The 0523 pump curves states that your pump will be pushing a little over 4 CFM. That's good for 3 or 4 diffusers.
If the diffusers are set at the bottom (therabouts 12')...those 2 diffusers should be moving in excess of 5,600,000 gallons a day (kep in mind they are overdriven with 2 CFM per head, so the lift calc is low). Not quite the 2X you are looking for, but very close if not over due to being overdriven. Adding a 3rd diffuser at the bottom would push more than 8,400,000 gallons a day if run 24 hours in that day. You would need to run this system for about 9.5 hours in a day to get one full turnover. Not Bad! The previous set of numbers is hard to estimate because the lift charts for vertex diffusers do not specify varying CFM's. Your 2 diffuser system would be overrunning the heads by about 2X, hence produce more water lift (theoretically). I assume the charts are based on their 1 CFM per diffuser recomendation. Adding a 4th diffuser reduces the amount of air that each diffuser sees which puts the CFM at the charts reccomended flow. 4 diffusers allows a more spread out placement in areas that you may want more aeration/surface agitation... 4 diffusers could produce 11,200,000 gallons per day. That's over 3X
Since I am spending your money...go with 4 diffusers with their own dedicated 5/8" lines from the manifold and the 3/4" or 1" feedline. Put two diffusers spread out in the deep area, one where it looks good in medium to deep water, and the last in a shallow for wintertime. I'd put control valves on each leg to ballace the flows as desired (run the shallow one in the summer with it choked back to flow like the others, otherwise it will boil like mad and the others will suffer). If you only run a single diffuser in the winter...you'll want to dump some air at the pump to avoid blowing up your single diffuser. I don't know if that is really a concern, but when vertex says the diffuser is rated at 1 CFM, 4 CFM sure seems like alot. An additional flow control dump valve would allow for the excess air to be dumped to the atmoshere.
EDIT: do the feild tests that Bill mentioned and compare to our calcs and post back some results. I like to be double checked.
