I'm wondering if Black might not have a slight edge, here. Black's prospects on the Q-side seems at least good as White's on the other wing, and Black's hold on the centre, especially the dark squares, looks the more secure. I'm pretty sure I would prefer to have the Black pieces in this position.

I think 16...g5 is the more circumspect: keeping the lines closed on that wing for as long as possible.

It is usually advantageous in 'opposite castling' games to get your licks in first. Who is first thrown onto the defensive is usually at a disadvantage. But circumstances alter cases. A defensive move like ...g5 would have slowed down White's attack, which needs more building anyway. Permitting White open lines helps White immensely. If Black still holds an advantage after this, it is probably only a marginal one.

18.hxg6, surely! Though Black would be doing fairly OK after 18.hxg6 Nd3+ 19.Kb1 b5 20.gxf7+ Qxf7 21.Bxd6 Qg6 22.e5 Nxe5+

A dream position for Black...

Simply 20...fxg6 looks OK, but safest seems 20...Bg7 21.gxf7+ Qxf7 22.Bxd6 Qf6 23.Bg3 b5. Black's hold on the dark long diagonal and more mobile attacking wing seem to confer an edge upon Black. Not to mention the extra piece, of course!