I use the 2/3A NiMh batteries for Ignition also,, you can get 1600 mah ones and the are very small and weigh about 3.5 oz
A 2s 2300mah A123 weighs about 6oz. then add the weight of the BEC probably 1oz
A 5 cell AA pack weighs 5.4 oz


You wont save weight with the A123 unless you use the BEC with it, you may save 3 oz vs. 2 packs, But you'll need a A123 compatible charger too if you don't already have one,, That's a lot of money to invest to save 3 oz