A guy I work with has clip-on aero bars. His main complaint with his current set-up is that he can't shift while in the aero position. He has to take one hand off the aero bars and reach for the shifters. Says its a little unstable steering one handed with the aero bars while reaching with the other.

I want to suggest moustache bars. They would get you a couple inches of drop. The problem is the ones I know of use road bike shifters and brake levers. Your existing ones won't fit. With my moustache bars, I can get a pretty good aero position with my hands on the brake lever hoods. Nitto North Road bars flipped upside down might aproximate moustache bars. They take MTB brake levers and grip shifters.