If you run an extender with the G2, you will need to change LA.
Those rechargeables you mention are 3.7v (ie you will total 7.4v for 2 and they'll actually be 8.4v hot off the charger)
Thus you will blow a 6v bulb such as the P60 or P61.
You need a 9v LA such as the P90, or a G&P or a Digilight.
How much longer run time someone else can answer as the 9v globes are brighter and thus draw more juice than the P60.
You could get a 3.7v LA and rechargables 17670's (using one at a time) and have a pretty good setup. Look for AW's thread in the Dealers section.
Regards
Dave