When running a red dot on a rifle, like an M4, your head is automatically positioned to instantly pick up the red dot as the rifle is mounted. This is due to the additional points of body contact afforded by a rifle (stock and handguard).
A pistol however, does not offer those additional points of contact. You will not use the red dot the same way as the rifle regardless of how tuned your draw is.
However, with the traditional placement of the iron sights, sights that the eye has been trained to seek through countless draws in the past, the eye will go to the irons initially as they have always done in the past. The instant the iron sights are picked up, BAM, there is the red dot, and the eye can let go of the irons and use the red dot...ignoring the irons for any further use.
The "visual hand-off" takes place and becomes almost automatic.
The irons serve as "runway lights" to get the eye where it needs to be to pick up the red dot. The traditionally positioned iron sights are analogous to the stock on the rifle for this purpose. Attempting to do away with sights, or to place the sights in anything other than the rearmost position on the slide is the mark of someone who has not thought the matter through, or is taking a manufacturing shortcut.
There are no shortcuts...there is one proven way to operate a red dot system on a handgun.