It kinda depends on how big your track is, and how heavy your trigger finger is. I get away with 20C packs just fine, they're barely warm at the end of a run. I let a friend drive them, same track, and they come off pretty warm, and depleted. My point it, there's nothing wrong with the 20C Zippy packs, IF you can control yourself, and you aren't on a HUGE track.

Here's a vid of the truck and the track for reference. Running the Tekin 1700 Kv system on a 5S 4900 20C Zippy Rhino pack:

[youtube]http://www.youtube.com/watch?v=6ZfMNFBy6AQ[/youtube]