Question asked, question answered. You drive your CX-5 like an old grandma and it's going to learn and respond in kind. Reset the ECU all you want, you don't change how you drive, how's it going to learn any different?
While this is true to an extent - lets not discount the fact that the ECU is always learning and that learning period isnt 5 weeks or 1 month or something. If he starts to drive aggressive the ECU should learn quite fast - its not like if you drove like a nanny for first 2000 miles the CX5 forever is stamped with a less aggressive shift style. Plus he says he feels his brother's drives better and responds faster. So he is the same driver in both cars.
To the OP - its hard to prove this case as a Lemon. Your best bet is to make a dealer service / technician whatever drive it himself / herself and come out with the verdict that something seems wrong. You can try 30-50 times / 50-70 times / record when it downshifts etc etc, unless the dealer or a mazda technician says so - no one is going to entertain this as a Lemon.