Yes, of course the shock wave is conical; I just highlighted above and below because they're the only directions likely to influence how quickly the bullet falls.

I don't think the effect would generate lift per se, assuming a horizontally fired bullet (since, again, it's symmetrical). What it does seem to me though is that it would effectively increase the density of air local to the bullet (in all directions except for behind), and so it would essentially be falling through denser air than it would be if just dropped from stationary. That would result in a small but non-zero increase in the time taken to fall the same distance.

Again though, that's just what intuition tells me; I certainly haven't tried to model it or anything.