I am thinking maybe a change from the factory pads will help?
The stock rear brake doesn't grab hard until it is applied quite hard. I can see how using it for sport riding could be a little sketchy. I think the stock brake setup is all fine except for riders who need instant grab for improved feel. The flipside is that the weak rear brake might discourage riders from relying on it because it can result in a highside crash if the rear wheel locks. I agree with Hub, the master cylinder is what needs to be changed or else just get used to all the throw in the pedal before it grabs hard enough. Racing people prefer instant grab so there is no searching through the throw for the right amount of braking.
I was looking at a Beringer rear brake MC which comes in three bore sizes. It can be equipped for a thumb lever or used with the foot pedal only. I think you can have both hooked up. I was planning to mount the Beringer MC to the forward muffler hanger bracket bolt. Use a longer bolt and position the upper heim joint on that to the inside of the frame, put a nut on the other side of the heim joint. It might work if the MC isn't angled back so far that there is a possibility of the brake pedal swinging all the way around. I guess you could always raise the brake pedal if it was necessary to prevent that or maybe you'd need to fab a bracket for optimum positioning of the upper heim joint. Optimum is probably almost straight up and down like the stock MC. It's safe at least. Not very fancy and not very powerful but it's safe for most riders.
'08 MIDNIGHT SAPPHIRE BLUE Now Deceased