I haven't used a receiver pack in a while, but I did in some of my first conversions. I actually ran a 2S lipo (say around 1500-2000 - generally still pretty small and light) with one of these Novak BEC regulators (they seem to have actually updated the design to be more lipo friendly with a warning LED so you don't overdischarge the pack):
http://www.amainhobbies.com/product_...ducts_id/23173
Running a 2000 mAh 2S lipo with this in a LST2 with upgraded dual - 250oz servos I would run for a good 2 hrs and not even drain half the pack.
And with the regulator you don't have to worry anbout the voltage dropping. If the regulator bust and you get the pack voltage, I've read that most receivers won't fry with about 8v going through them.