That extra mah has to come from somewhere, and in 18650's, that means they're a shade taller. Taller
batteries will compress the spring too much to make it a reliable safety (the spring collapses in case of a short to make the battery drop away from the firing pin, preventing further damage to the unit). Plus, the AW IMR's are recommended because of the way they drain, and they're chemically safe so no in-battery circuitry to fail on you either. The 1600's last a good 12+ hours of normal use, the extra hours aren't worth it (in my opinion) to lose the safety features of using the recommended battery.
Now, the extra voltage you get with 4.8v's... that's worth losing the chemical safety and drain rate in my book - DRASTICALLY shorter life (only 600mah), but it still maintains the spring safety.