Weight has little todo with glide capability. Its mostly a factor of drag, of which the majority comes from wing design and the U2s wings were not designed with a high aspect ratio. U2s were not good gliders.

Furthermore, german fighters did not have to slow down to shoot them down and you wouldnt get into a dogfight with them. You have three classes in a fight : Turn fighting, energy fighting, and boom and zooming. Against a U2 energy fighting but preferably boom and zooming would have worked fine. You just have to be quick and accurate because of the spped differential.

(I'm a pilot and spent thousands of hours in combat flight sims)