I just bought a Zippy 6s 4150 R pack for $130 shipped, I havnt run it yet but they use Korean cells in the R packs so I am hoping for close to FP performance. Are you looking at having more of an on road car, I transformed my Rustler into an on road car and you can get over 60mph, mine on 4s is around 70mph, they dont handle that well especially at that speed, I also have an 1/18 scale RC18 on 3s that is pretty fast I am thinking about a 4s upgrade......:-)
this is a link to the R pack I bought, they are also available in 2 and 3s
http://www.hobbycity.com/hobbycity/s...Ah_21-35C_6S1P