I don't know if you are still under CPO warranty, but if you are the dealer had to certify Brake Pad Thickness (6 mm pad material min.)and Brake Rotor Thickness (rotors 50% of wear limit) this is from the CPO Brochure at the time of the sale. In any event the wear limits of the rotors are stamped in the edge of each rotor. So you can measure yours against that to find if you are indeed in need of new rotors. As for the wear indicator it is worn by the edge "lip" of the rotor, so the more worn the rotor and the greater lip will trigger the low pad warning while there is still a lot of pad material left. In my experience of over 100k miles I find the rotors last for 2 sets of pads. On x351s the rears wear faster than the fronts as the rears are used as part of the handling setup and "drag" in turns to hold the rear end in and track better. Recently I had my dealer replace all 4 rotors (OEM and price matched form Jaguar Parts Unlimited which is the web presents of Exeter Jaguar in New Hampshire) and install Brembo pads I purchased through Rock Auto or from Brembo's site as I was changing pad material and wanted to start fresh.. Stopping is great and pad dust is way way down. Lastly my dealer will only recommend rotors when the are worn below spec.

 

Yep, I put Power Stop Evolution Z16 ceramic pads front and rear from Rock Auto on my old XFS and they were great, in fact the best brake pads I have ever had on any of my Jags. Very cheap, good stopping power, zero fade, good wear, zero noise, nice and progressive and best of all almost zero dust.
The only problem is they don't make 'em for the XJR etc with the 380 mm front and 376 mm rear brakes, only for the "cooking" models with the 355 mm front and 326 mm rear brakes.
I have also bought parts from Summit Racing over the years (although not for the Jags), good prices and service.