You'd get about 1/3 to 1/2 more torque. Top speed would stay about the same.
About $600. $400 for a NPX controller and 200A solenoid and $200 for a complete set of top quality 4Ga cables (all 13 of them)
http://www.cartsunlimited.net/Custom_Options.html
I'd replace the cables first. You can find cables cheaper elsewhere, or make them yourself.