Considering the size of your system I understand why an external pump makes a lot of sense. How big of a pump are you going to upgrade to?
Have you had any issues balancing the flow to your lower tanks vs the flow to your upper tanks?
I am looking forwards to seeing your DE implementation! I think you will really appreciate the DE... but it is easy to get spoiled quickly and forget what cloudy water looks like. I still haven't changed the original DE powder on mine and it is still going strong!
This setup is so awesome.
I dont know if you have already answered this, might have missed it on the read through, but are you using 1 in. over flows as well as 1 in. returns? Awesome job, looks great!