When I had a
vertical overflow into the sump, I tried a U-tube. Tried it right at the water surface, below, all the way to the bottom then all the way back up. I personally found it made no difference, because in my case the problem was just the sheer weight of the water in a straight vertical column. I also had issues of the overflow "burping" (pressure building up then suddenly releasing). It was horrible. The only thing that solved this was having the overflow go nearly horizontal at one point, and then "slide" into the sump at a 45 degree angle. I can't beleive how much of an improvement that made.
In my case though, was a standing pipe type overflow in the main display, and the water drained downwards into the sump in a perfectly straight column. Definitely not the way to do things...
PS. Edit. Apparently I do not know the difference between "horizontal" and "vertical." Ooops. 
ops: