I only load my top case with Bakery Items.First, google "fjr subframe cracking" for a look at the sometimes problem.
Then the thread: http://www.fjrforum.com/forum//index.php/topic/154924-gen23-subframe-stiffy-kit-by-garauld/?hl=+subframe++crack#entry1099430
Touratech says the weight limit for their Yamaha luggage rack is 10 lbs. I figure that is a function of the subframe weight limit. If you are going load up a top case, the stiffy kit might be a pretty good idea.