Well, I work in the automotive supplier industry and we are selling the same product to the OEM as we sell to the OEM- and independent aftermarket. I would think that this is common practice. I just can't see that a supplier would make two different quality products of the same part. Most OEMs are getting their OEM replacement parts through the same supply channels as their production parts.
For products like tires, brake pads, clutches etc, which are "wear parts", the aftermarket quantities are significantly higher than the OEM numbers which would make it not very economic to make a more expensive part than for the lower quantity OEM business.
As far as tires go, the only difference I can see is the age. All tires should actually sit a little after they are being made. If the just-in-time process gets them too quick on a vehicle and after that too quick on the road, the fresh rubber may wear prematurely. But that is really splitting hairs.
You guys really think, that Dunlop makes a 501 with "Harley-Davidson" molded on them for the factory, one with a different rubber mix for the dealers and again a different one (or two) for non-Harley applications? You're kidding, right?