I only have experience with the MRC-3 as for out of sump skimmers, and it is above the sump. I would think that elevation, relative to the sump, isn't a big factor, as far as function goes.....since the pump will dictate the flow of water into the skimmer. However, most out of sump skimmers need to have a gravity outlet which becomes tricky to plumb up with the skimmer below the skimmer.
In case I lost you on it, the pump controls water flow in, but a gate valve will slow the water exiting. As water height in the chamber increases, pressure forcing down increases...and the gate valve combines to allow you to adjust water height in the chamber.