How are the tires? Are they the normal loader type? By that I mean big blocks of tread? How about grooving the tires to add more biting edges? I bought an IDEAL tire groover from here: http://www.bryke.com/wheel_tire_accessories.html
You can get up to 3/4" wide head and blades to cut a more appropriate size groove in these big tires. (or even multiple passes to widen it more) These will act like sipes on smaller tires. It will likely help the tires run a bit cooler. I've noticed that loader tires will loose a bit of traction as they warm as compared to cold tires.
Adding ballast to the tires will help some, but it will definitely affect the acceleration and braking as well as top speed. But grooving will assist traction, braking and steering to a greater degree, IMHO.