yeah, 500 grams is nice... my 7s A123 pack was nice and light around 550 grams, and I noticed a difference between that and a 7s2p pack, but had moe power, the 7s pack made the Revo feel more agile for sure though.
I suppose 4000mah pack running 6s will run pretty cool anyways, so yah, why not go for something lighter (and cheaper).
I would like to try out 12s A123 someday, two 6s packs on each side would be nice... then get a GM single speed tranny to free up some room. Only thing, my 'monster' Quark 125 can't do 12s.
In the end, AAngel, it's all up to you.