I've never seen a banshee with a roof ingame.
EDIT: Ok, nevermind. I did some tests and actually I am able to kill people on banshees, be it on the top, left, right, etc.
However, tests also did show that Stallion with the hardtop (on both sides, just on the victim, just on the shooter, does not affect if you can either kill him or not) that you can kill him from the top even when it has a hardtop. The same happens on SuperGT.