would the height of the return effect where the water level is in the skimmer
Definately. The water is not free flowing out of the gate valve. It is acutally having to flow (be pushed) what looks like to a level above the neck of the collection cup from the picture. This is going to cause massive back pressure since it is taking pressure to push the water up to that level rather than freely flowing out the gate valve.

I actually always thought that the outflow of these skimmers was supposed to be above the water level in the sump to avoid the little back pressure that can cause also. Anyone have any other info.