Assuming you are using it for the return on the 125 I would say that's pushing it as far as return volume but that's JMO. If you setup the penductors in that config you will get more in tank flow out of them with penductors but not a ton more....maybe twice the flow....Mags don't handle head very well. If it was my setup I would run 2 returns out of the Mag straight into the tank, possibly just one on a sea swirl or wavy sea or something similar. I would then run the OM unit on a closed loop with a pump in the class of a GenX pcx 40. I have a Genxpcx 40 running a CL with 2 penductors and I would guess I get more than 4000 gph out of it.
hth, Chris