My sunglasses are useless when cycling, because with my head down, I end up looking over the top of my sunglasses below the peak of my helmet, with the sun in my eyes.
Thats the problem with most sunglasses made for all-round use (and for some made for cycling).
I have Briko cycling glasses that covers the gap 100%. My Adidas Gazelle is also 90% perfect when correct adjusted. (I tried Adidas Evil Eye, made for cycling - they didn't cover the gap.
I, too, have Briko cycling glasses and have no problems with the sun peeking in over the top. They came with three different lenses for around $125. They have been worth every penny.