With an analog BEC you are throwing away excess current to drop the voltage to the required levels.

It is unlikely with this combination, that you'll get additional run times with a 1300 11.1v 3S LiPo pack versus a 1400mAh NiMH pack, provided that both are in good condition.

If the latter is aging, while the former is relatively new, then yes you may.

Now if you were using a switching BEC ( aka a UBEC ) it is entirely possible that you may indeed get better run times, as a switching BEC may not place as high of a load on the battery.

However I'll go with UBECs or BECs only when there is a clear advantage to doing so...

e.g. I can either cut weight, or install a much HIGHER capacity pack for the same weight penalty...

So I may opt for a 3000mAh 7.4v LiPo pack to power the electronics using either a UBEC or Regulator, knowing that I'm only adding the equivalent to a 1400mAh NiCD pack in terms of weight.

In your shoes, I'd just buy another bigger battery pack...

e.g. one of these at a fantastic price: [link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXVHY4&P=7]3000mAh 6v pack ( click me! )[/link]