My guess is that when you have OEM glass even if its not hud stamped it's probably pretty comparable. But as soon as you throw on an aftermarket glass it wouldnt surprise me at all to see a difference as stated above.
I installed hud in my 06 and been in an 06 with a hud windshield and could not tell the difference.
I myself can notice sometimes that non oem windshields tend to be more wavy than oem.