Ya your right Tim I would be screwing up just like you would if I put a mag on my truck hood and just drove around waiting until your plastic fantastic mag well melted to the hood!
But the fact is on a 100 to 110 degree day in AZ the interior temps reach over 150F sometimes. When I store my gun in the gun bag in the back seat of my truck and stop on the way home from work to practice or do what ever with the gun I expect the plastic parts to be intact and not be so soft that they fall off the gun when removing from the soft case in the back seat of my truck. At this they wont stay on the gun any more as its so deformed at this point that it goes into the garbage can. I never had a Magpul plastic part deform as its in the same bag on the gun with your magwell etc... Quality parts are tested to work in all environments not just in 80 degree heat and lower.
BTW I have been shooting bottom feed shotguns for years with 2 Plastic mags coupled together with a Aluminum Coupler and never had a issue with the sun or with dropping it on the ground during reloads
So I guess the MBX extensions really are overkill too as Aluminum to plastic is overkill. I wish MBX made a extension for the MPX mags as there stuff has proven to be a winner so far with the Glock mags.