The track on my Shimano rotors are about 15, maybe slightly over, at their widest part. The area swept by the pad is a little bigger, they cover maybe one or two mm of the tops of the arms. They are not XTR ones but my bet would be that XTR ones would have bigger tracks if they were different at all.
You were only ever going to get one response from the manufacturer, you'd get the same one from any of them. If you asked a car manufacturer if it was ok to use after-market brake pads in their cars you get the same answer from them too. 'Well, it might work but we can't guarantee it and the only way to be sure the brakes are going to work as we designed is by using our pads'.
They have to say that. Firstly, they don't know which brand of cheapo pad you might stick in there, so genuinely can't say how it will work, and need to cover themselves legally. Secondly, it's their job to sell product. They don't want you to buy a cheaper product from another manufacturer, they want you to buy their product. It's obvious but people forget. Never take at face value the opinion of someone who has a material interest in the decision you make.
I used
EBC Green Stuff brake pads in my car and they were better than the Nissan pads I have now. I'll be buying them again next time. They work better than Nissan pads, despite what Nissan might say.
As I said in my first post though, I wouldn't buy them without checking more. I've phoned a couple of good bike shops on your behalf. One said that it's best to stick to the same manufacturer, if for no other reason, because that way you'll have some sort of comeback should you experience any problems.
I've spoken to two separate people at another other shop who said there is no problem. One guy has run different disks and brakes many times without issue and says that the rotor sizes are pretty much standard. He says that there are slight differences in the thickness's of rotors but the brakes are an open system and self-adjusting so it's not a problem. He says that most rotors are of a similar thickness anyway. There are exceptions, like older rotors that are very thick and Avid ones that seem to be thinner than average, but generally they all fall comfortably within the spec of the brakes. He also pointed out that there are after-market companies who make pads and rotors for bike disk brakes, I'm sure EBC are one?
Why not speak to a couple of shops yourself and if you're encouraged by their response you could buy oner disk to try out?