does anyone use really big batteries?

[VintageMA]

Take a look at the Neu 15xx series page here:

http://www.neumotors.com/20061222/1500_series_.html

Lower winds = higher kv.

On 6S you are probably best with a 1515 2.5D (what I run in e-Revo on 6S - just over 35mph with very moderate gearing and no heat issues - I could gear a bit lower and have it going 40-45 easily) or a 2D. 1Y may work but you have to gear pretty high and the first few minutes of a fresh pack are going to push that motor up to about ~55K rpm - I think you'll have heat issues.