Danny
I haven't heard of anyone reporting a bad rear disc,any problem seems to be on the front only (just ask Adey)
When I was on my US trip, had to use different dealers for service etc according to where I was. None had problems putting on EBC front pads rather than Trump, one even fitted sintered without my knowledge (hated the pads by the way)! At no time did anyone mention that there would be a warranty issue if the disc went bad.
When I got my latest bike from Hughenden M40 (Thame Oxon) not only did they fit TOR's for me, but they also took out the AI system( I supplied the kit) at the same time! Again there was no mention of warranty issue when they fitted the AI kit, (but also not shown on invoice)
Don't know about service cost as haven't got that high yet, but the interval has been changed to 6000 miles so if you can do the pads yourself, maybe you can delay the service for another 1000 miles?
Take it easy
Shaun