The farther away the optic is from the bore, the more difficult it will be to get a zero for the optic that will be useful for more than a specific distance.  This is because as you increase the height of the optic, your line of sight is parallel to the bullet's arc for less and less distance.

Another issue is that you can't get a repeatable cheek weld when your optic is 2" too high.  And even a slight cant of the rifle is going to cause large errors in aiming due to the distance of the optic from the bore.

The only type of optic that is really fine with being mounted on a carry-handle upper is a "reflex" type optic like an AimPoint or EOTech, and these are forward-mounted on a gooseneck at the same height as the iron sights.

-Troy