I used the KArace 8.7 2013 model on rs5 122 and it is not too big for the board. Wind around 13-18 knts, fin Sonntag 45, choppy conditions. I found the sail felt a bit heavy once getting overpowered, but probably I need to work on the tuning to get the best out of it. I heard that 2016 karace sails are a lot ligher to handle, but have no direct experience.
i also used that sail in lighter winds with bigger boards, all my sessions with the KARace 8.7 are here:
martignoni.id.au/sessions.php?sail=KA%20Race%208.7 Later years Koncepts in my opinion are much better and smoother, but the larger sizes are cut very flat, no bottom end power.