Do you have a PVC cap on the top of the standpipe with a hole drilled in it like a standard durso? Looks like it is completely open? Wouldn't that allow too much air, and a lot of gargling. That would slow down the flow if there is a lot of air in the tube imo. Can't exactly tell from your pictures. Can you post up some better shots (close ups) of the overflow plumbing?
See post #10
http://www.reefcentral.com/forums/showthread.php?t=1276584 . Looks like you do not have the cap/vent? Maybe I am blind.
All that electrical is very clean. Looks great.