If it's built right, you should be able to get about 300 GPH from a single overflow, which is 7.5 times turnover per hour in a 40 gallon tank. Just be sure to test it (two 5 gallon buckets of tap water mocked up at the tank/sump height works great) before installing it to be sure of the flow rate, and then adjust the return pump to match the flow.
Another option is to setup up a venturi on the return pump connected to the hob part. To suck out usually a little water, an occasional air bubble, and the air to reestablish siphon when the siphon was lost.
If you have an HOB on your tank, it's an excellent option. Unfortunately, my return pump is in the fuge about 8' below and about 12' feet from the tank. I have 5 pumps crammed into the return chamber, so there's absolutely no room to install a venturi. I believe a $20 lifter pump is my best option... but the venturi is a really good idea for those with HOB's.
I was thinking of doing the exact same thing.
I must be missing something here but then that's the way I learn.
Many external overflows do not lose siphon when the return pump stops. Since the siphon is still active it can pull water through the venture tubing for as long as the intake and outlet of the powerhead are submerged. Once the water level in the tank drops below the powerhead the venture will suck air and break the siphon.
That's what I thought.
1: 125 tank, 2: 40B tank, 3: RDSB, 4: GFO/carbon reactor, 5: UV sterilizer.
I sold 4 practically new Koralia Evolution 1150 pumps for that reason. Pumps using A/C current are notorious for starting backwards. Fortunately, all 8 of my powerheads (in 2 tanks) are D/C powered, so they always start in the right direction...
I suppose once all the plumbing is done and everything is dialed in with an external pump, it would make life a little easier. Getting there would be the challenge. I bought specific sized pumps as I knew what the flow would be for each system it provided flow to. Splitting a single pump into 5 outlets and tweaking the flow for each tank would be a bit difficult to do, since the refugium is in the basement and the tanks are upstairs. The RDSB, GFO, and UV wouldn't be bad since they're in very close proximity to the fuge. I'm sure it would also reduce the heat produced by those 5 pumps... though my tanks never get above 81F.