I use throw over panniers (kindly donated by my mate who forgot I'd borrowed them!) but I wouldn't fancy letting them rest on the plastic exhaust covers. For one thing I reckon they would rub and make a hell of a mess nevermind melting.

I fitted the yamaha top luggage bars (in case I can ever afford the side cases!) but they do prevent the throw over panniers from touching the exhaust covers and in the event of an off road spill they provide a bit of protection to the exhausts.