If the effect would be neglectable, de Havilland would not have bothered to install a fixed slat on the inboard side of the "external" Comet IV wing tanks. Those tanks have a significant influence on the wing region inboard of it. This is the same for tip tanks, those are very low aspect ratio very thick airfoil wings. They produce additional local AoA in their vicinity (their "tip vortices"), roughly they double local AoA. It definitely increases lift in the tip region, especially at high AoA. How much that matters for the complete aircraft is hard to determine.
Last edited by Volume; 1st Oct 2013 at 07:30.