If you are experiencing rear brake fade, why not just get a brake line cooler from Slavens Racing. You don't really need more rear braking, you just don't want it to stop working when you need it. A cooler will help keep brake fade to a minimum if you are using it a lot.
Otherwise, like Willh said, the twin pad rear caliper from the 990 adventure, SMR or SMT will slide right on the SD. You will still have the same issue with brake fade though once the fluid gets hot.