I believe all this backrest discussion to be effected by the recent subframe cracking threads. Just like putting to much weight in a topbox, and overloading the design of the subframe, any backrest mounted to the 3 bolts, or in the case of the PCA rack mounted to that rack, will produce a load to the subframe leading to all the cracking problems. The Pirates Lair backrest mounts to these same 3 bolts, and from the looks of the pic of the Garauld backrest, it appears that one does too. All this of course if the passenger puts any force on the backrest while in use, but that's what its for is for them to lean back against it. The amount of pressure a person could put on the subframe by leaning back with any amount of force would be pretty great.
My subframe isn't cracked yet, so I will be replacing my PCA plate and backrest with the Give SR357 mount, but then would need to look for a backrest for the passenger, so the option of the Sargent seat/backrest combo is an interesting one if indeed you can use it with the Givi topbox in place.
Do a search for the subframe cracking issues to learn more.
