When deciding to add the red dot to my Hellcat, that was my #1 requirement ...... to co-witness 100% with the irons. Now on a long gun a lower 1/3rd will work pretty well since with the longer sight radius you can simply tilt your head just a tad and get a reasonable sight alignment and picture. With a hand gun that doesn't work nearly as well.
My preference was that when I come up on target, I want to see that red dot first, then my irons if need be. But the whole point in adding a red dot is so that you aren't required to see the two sights at the same time. If you can see the red dot on target, assuming it's been sighted correctly, that's where the bullet is going (POI) because that will be your POA. By the same token, if you can see the irons, even without seeing the red dot, (which isn't likely if they're co-witnessed 100%) your POI will still be your POA with those irons.
As an aside, as many folks as there are out there who would love to put a red dot on their pistols but have been unable to find either one, if you later decided you didn't like whichever one you found, you'd be able to sell/trade it PDQ.
One last note: I'm not certain about the weight constraints for a red dot on the XD's, but it is a serious consideration on a Hellcat (limited to approx .5 oz). Be very conscious of what the dragonfly weighs. It may be too much for the XD slide to work properly. Good luck in your choices !!!