I've used the same 4 cell li-on battery for over 2 1/2 years now of frequent trackdays and racing (this one:
http://www.ballisticparts.com/produc...ries/4cell.php)
I've had zero issues the entire time. Either I have a magic battery, or your salesman is talking out of his ass
For what it's worth, I don't know anyone who's running an 8 cell in a baby ninja