Unless the mesh dish is very very old, I'm confident that the mesh is small enough that it is 99% as efficient as a solid surface. But as the above responses mention, shape of the dish is really a critical factor, and it's very common that a mesh dish doesn't have very good shape. {Actually, the shape should in theory be an elliptical shape, not parabolic, but this is a trick statement, because at these distances, they're essentially the same.}
Also, as mentioned above, having the focal distance set properly, is very important, because with Ku, if the focal distance is set wrong, the signal pattern will look more like a donut than a spot.
But the main reason for responding is that even though most C/Ku feeds don't illuminate well for Ku, and the dish shape may not be perfect, and the focal distance might be off a bit, since the Ku sats are generally more powerful than the C-band sats, and since a dish theoretically will have a higher gain on Ku than C-band, and since a big dish will generally have more resolution to reject adjacent satellite interferrence, you'd really expect Ku performance to be better than C-band, unless the dish is really in bad shape.
In MY case, when my 10' mesh dish was new, it performed pretty well on Ku. Significantly better than a 3' solid dish, and much better than a 6' mesh dish I have. However over the years, it has gotten dented and warped, and it now gives extremely poor performance on Ku. I've tried all sorts of things to get my dish back in shape, but so far nothing's worked. I think that when the weather gets better, I'm going to pull out some of the mesh panels and see if I can flatten them, but I'm not real optimistic that this will help. I really think that I need a new dish.