Your pump does closer to 700 GPH
with a 4 ft head, do you have it restricted with a ball valve? If not your overflow is being pushed to the limits.
With Under Over Under, you'll have a section that is between those two outer baffles where the surface water grows stagnant and algae can build up.
With Over Under Over the only disadvantage is if the water level in the pump section is too low it creates a waterfall effect producing more bubbles but that can be avoided with an auto-top off system.
Your sump is more then large enough for a lighted fuge, have you considered it?
for sump/fuge ideas.