Running the same numbers (except tubing size) as I used earlier on the linked site yields only a 3.6 psi drop WHEN using 3/4" id tubing instead of the 5/8". Quite a bit of a difference.

Don't let me come across as an expert on this subject as I only dabble in Fluid Dynamics on occasion.

Given:

1725 motor RPM (60 hertz US electrical supply),

300 feet of 3/4" ID tubing,

The equivalent of 4 tees and 4 elbows somewhere in the system (my estimate of your manifold design which would be similar to another 30 feet of tubing gathered from here...

http://media.wattswater.com/orion-hp-frictionloss.pdf

),

10 CFM Air Flow,

And 15 PSI max psi at pump,

You will see a 4psi drop from the pump to the outlet (sitting on the driveway and no diffusers.

Put it in 6 feet of water and add the diffusers and you will loose another 5 psi for a total of 10 psi.

Using your pump curve...you would be producing 12 CFM.

I would say that you are onto something.

If you use a true 5/8" ID tube...you end up with 15 psi and you still end up with just over the 10 CFM you specified, but you have to consider all the "assumptions" in the online calculators that I have used. The effect of these assumption could be good for you, or bad.
I tend to be more conservative and assume it would be on the bad side. This 5/8" scenario seems risky to me and going through the "hair pulling calculations" that Journey mentioned is outside my patience limits. It's funny how engineering calculations, so often, end up on the cutting edge and true experience ends up being the deciding factor. To quote the experienced (Bill Cody)...

"3/4" is probably better" and I'll add "definitely better".

The only concern is IF you end up with too much flow that could damage the diffusers...You might have to add a seventh diffuser or a pressure relief valve so that you don't "pop" a membrane (or wear them out prematurely). I have an adjustable pressure relief valve in my system for when I just run one of the three diffusers during my winter months. It merely exhausts (wastes)some of the air to atmosphere so that the sole diffuser is not blowing up like a balloon. I adjust the relief valve and watch the pressure gage so that I acquire the pressure that correlates with the pump curve to get the CFM that the diffusers call for.

Fish on!,
Noel