I think thats wrong Rockerpeller- if the overflow isn't siphoning, then it wouldn't matter about the size of the return- only as much water will go down the overflow as the amount put into the tank.
How big is the tank SilverBlade86? That AT Pump you posted is only running 1500LPH at 0 head. The AquaOne CF1200 external does 1200LPH at 1 meter head or something, that submersible pump is not going to do that much, on a 3ft tank or above I would really recommend getting a decent second hand Eheim or something