Seen a few rear sub frames cracked or broken. If the weight is close to the seat, then you could get away with it. Also depends on road conditions. My BMW warning label on the luggage states a max of 120km/h when fitted. That Lamonster rack, as pretty as it is, is a recipe for an overloading disaster.
Went riding with my wife today, I have the lamonster rack, now the taller backrest for her, wondering about the same. I guess we need to find the breaking point.
Oh well, first one to break a subframe, please shout.... timber!