AGA Megaflows. To plug or not to plug?

[WinnipegDragon]

I'm still in the planning stages of my 120g, but I was wondering about the 'slits' in the AGA Megaflow overflows. I don't want to hotlink someone else's image, but you can see a picture of what I'm talking about here if you haven't seen them before.

Basically, it's designed to draw water from three levels in the aquarium but I'm concerned that this might impede surface skimming if there is less water draining over the top of the overflow:



So, should these be sealed up or not sealed? I'll be using the durso-style standpipes so accidentally draining the tank is not a concern.