I was told if I want to carry weight with my F550 then get 4s, I bought 4 of these 4500 4s nano tech batteries.
http://www.hobbyking.com/hobbyking/store/%5F%5F11932%5F%5FTurnigy%5Fnano%5Ftech%5F4500mah%5F4S%5F25%5F50C%5FLipo%5FPack.html
I was using 10" stock props and just picked up 10"...