Modern Rear break conversions...

All the problems im having with my GS's stock rear system is making me wonder if anyone has done any rear brake conversions on their cafe? Ive been looking on ebay out of curiosity at Gixxer rear set ups, and have found that alot of them are axle mount calipers. Has anyone attempted such a conversion and if so what issues did you run into? Spacing and rotor coverage issues? Info would be great!
Thanks!
Josh