with overflows, you want the return pump to be pushing just a little less than the overflow can take out...however imo 600 gallons per hour less than the overflow can pull out may be problematic...you may lose the siphon. But in any case, it will be noisy. I'd put another 500gph on the return...but you dont want them rated to the same...well..once you calculate in the head loss from the pump, then it would actually be less than the overflow if you got a pump rated to the same as it...so that might work now that i think about it (getting them rated to the same that is...)
hth
mike