OK, I've done a bit more sleuthing on this issue. I was in to Costco this morning to speak to the head of their tire dept (whom I know and trust) to get his take on oem specs compared to after market specs for the same tire. I questioned him about the tread depth difference which he said he had not heard of, BUT, agreed that tire manufactures commonly change the specs of their oem tires(composition, side wall thickness, etc) to get a price point that the car manufacturers can live with to get the particular kind of performance they want the vehicle to have off the assembly line.
As erhayes suggests above why would you go back to the after market tire (Toyo A23 for example) if you hated it when you first got the vehicle, even if you were told it might be better in it's after market version.
Or as brillo54 says above, find a good replacement tire that wasn't used as an oem tire. This is my dilemma... I'm moving on from Toyo A23's, regardless of how good their aftermarket version might be, mainly because of it's outrageous cost, but can't decide on the best (not perfect) replacement. I keep reading on the CX-5 forums about a version of Continental, Pirelli and Bridgestone but there is no consensus on which is best, based on those who have tried different non-oem aftermarket tires.
Even resolving the speed rating issue is ticking me off as well. Tire rack tells me that Mazda USA told them that anything above an S rating is ok, but Mazda Canada told me that I must stick with a minimum of a V rating as in the oem 19" Toyo's. Yes, I know I will NEVER be going 130mph(H rating) or certainly not 149(V rating), but as one dealer asked me, who do you sue if the less expensive H rated tire you decided to buy blows up and the tire manufacturer chooses not to replace it because your model of CX-5 calls for a V rated tire. WHAT TO DO? UGH!!!!!