Put 15kg on the back and the bike doesn't handle any differently!
Wow! Is this using regular panniers or the Moulton-specific rackpack?
Thats with an APB rack. The TSR rack isn't as strong as the old APB one but the load limit for the rear rack+specific, Carradice made, rackpack is 16kg.
I don't think you can put rear panniers on a TSR. You can get a front pannier rack that takes small / universal panniers (I have Carradice overlander for mine). However, due to the long head tube, these front panniers do not sit on the wheel or move with the steering, so don't affect it nearly as much as low rider front panniers on a normal touring bike.
There's a picture somewhere of someone using an APB to move a chest of drawers, but I can't find it at the moment!
Instead, here's a picture of a fully loaded Moulton
linky. I certainly wouldn't take so much stuff, though!