Look for a Leopard 4082 1600kv & you can run 6S. They are sold from Hong Kong on ebay. Free postage too!
The larger the diameter & longer the can the more torque it will produce over a smaller size can with the same kv. So. don't just think a lower kv will improve things without taking into account the can diameter & length.
It is smooth canned & will fit a Losi mount.
I use this in my 1075mm Cat boat running twin 6S in parallel. It goes ok.
http://www.youtube.com/watch?v=eJO5N0Wh4eU
Losi SCTE SC4 Pro 4000kv, Tekin RX8, 2S
MP7.5 Kanai III Tekno Neu 1515/2.5D 1700kv MM 4S
LST 2 Carbon CC 1717 1Y 1580kv MM 6S
Futaba 3PKS Spektrum Pro