Yeah DEF definately modifier for Cannonball
Did this as 75BLU/THF with 300% Chain Affinity + Sneak Attack with around 760 DEF
(Taco, Reactor cool, Cocoon, Protect4)
http://img107.imageshack.us/img107/1766/cannon1ts.jpg
I did this 2x in a row, both for 1731. So I guess that is cap with my defense.
With BLU/WAR max at 300% TP with Chain Affinity + Defender with 878 defense did only 1106 on a spider, so it seems even though with /WAR you gain +100 defense, the Sneak Attack power from /THF gives a much higher boost