Talk to Purpony about getting an adapter to mount a Brembo P32 caliper to the rear of the bike. I did it to my XB12SS and I was able to reuse the existing brake hose by rerouting it. You can also use a P34 caliper, but you will likely want to use a new master cylinder as well.
http://www.mcmoto-worx.com/buell-xb-series-parts.html
I did it primarily to hide the caliper on my bike, the braking power increase was minimal with the P32. My $0.02 is that having a weak rear brake is actually a benefit as it will prevent locking up the rear under hard braking. YMMV.
There are also a few threads about this on here... a search should net something with AZmidget and Cooter discussing MC cylinder size and ratios.