Saltyfish, I'm curious after seeing your troubles with the k7 pumps since I'm in a very similar boat with my new system... I at first had similar problems with my four k8 pumps not staying where I aligned them (they would slowly point downwards, as you described) until I took them out to see what I could do about it. I realized that the little post wasn't all the way inserted into the pump housing which changed the center of gravity and made it not stay in place. You have to REALLY push the suction cup base into the pump, almost to the point where the pump is sitting less than 1/4" from the swivel ball / suction cup base. Once I realized that, I haven't had these pumps move at all from where I point them.
I was aiming for 50+ times turnover in the display when designing mine, but if you're having so many problems with your sand then I might be in for some annoyances other than silt clouds once I transfer everything. I decided not to do a closed loop because I wanted the tank to be completely silent with all equipment running downstairs, and wanted the energy savings plus very high flow of four Koralia 8 pumps to be able to accomodate whatever SPS I wanted if I steer away from a mixed reef. Since there's so many similarities between our two systems I'm going to keep an eye on this and use your experiences with your setup as a lab rat / sounding board for mine since I'm a month or so behind you in progress