I too am in the middle of a plywood tank build. (my
thread ). I now have the tank up and running in the garage doing leak and flow checks. I chose a Herbie style with a durso as the 2nd drain. I ended up with 2" for the overflows and am running 2 reef octopus S8 pumps. The plumbing for the returns is 1 1/2" with one of the returns terminating in a 1" random flow generator and the other return splitting into (4) 3/4" lock line sprayers.
My experience so far:
The 2" overflow drains can move a RIDICULOUS amount of water. like crazy amounts (It will drain my overflow box in about 5 seconds). I have the gate valve at about 1/3 to 1/2 open and the durso drain is just trickling. I've been experimenting with different valve positions and pump output settings and I cannot get the water to even overcome the overflow box. I figure with both pumps running full out, with head loss and everything I am probably around 2000 GPH which is about what I was shooting for. With the amount of water that the 2" lines can take, I would think you could easily run 3500-4000 GPH out of a 2" Bean Animal without breaking a sweat. One concern I had when going with the large plumbing is the amount of drain back I would get with a power outage. I have check valves on both return lines and when I shut the power off, I figure I get about 15 gallons that drains back into the sump. Most of which is from the 2" drain lines. (my tank is in the house and the sump will be in the garage so I have about 5 feet of 2" pipe on each overflow to go thru the wall).
I'm not an expert, this is my first large tank, just giving you my experience so far