P220C,
You are correct about more than needed for those ranges on average, but I'm reading that this shooter is not average. Sometimes you need to live with a few extra ounces to be able to see. Also, the added objective size shouldn't be a problem due to the height of the AR handle already acting like high rings.
Poor vision = needs for magnification and light
I agree with you that the shooter needs good glass.
I just find that a smaller, high quality, low magnification optic can transmit just as much light, and present a great sight picture to a shooter with slight vision problems.
Unless the shooter is nearly blind, he won't need 9x to shoot out to 150 yards, and a 3-9 won't focus in a home defense setting, so that really doesn't pass the test for this particular dual-purpose application.
If the optic is being mounted on a handle and not on a flat top upper, than the problem with large glass just got compounded, because the cheek weld is not as repeatable that high up. I wouldn't want anything higher than an ACOG mounted to a carry handle in a defensive carbine.
Remember the OP wants a dual purpose optic. This has to fill the hunting role, as well as be reasonable fast to acquire a sight picture and just beyond bad breath distance.
We need to help the OP to balance the needs of hunting with his defensive needs. if we lean too far one way or the other the optic becomes mission specific, and looses utility on the other end. A 3-9 x 40mm has no place on a home defense carbine. Not only is it sheavy and slow to pick up a sight picture, but paralax is crazy at close distance if you move your head. (I have moved groups 5" at 25 yards, and not really moved to any crazy degree).
Following is my ranking of up close sight picture acquisition speed. Obviously with enough training any of them can be utilized with good results. From 10 yards, with a slung rifle I can break a small clay target <1.5 sec. The advantage of the red dot is that you can do that over and over again. I'm running irons on my home defense carbine with a surefire duct taped to the foreend, and do not feel that i am at a substantial disadvantage. I have a 3-9x40 sitting in the safe that I would never consider using on my defensive rig. I haven't used the shot timer on the scope inside of 5-10 yards since you can't even get a sight picture at that distance: it won't focus.
Slowest- 3-9 x 40 crosshair
Slower - 3-9x40 illumitated dot
Moderate - Iron sights
Moderate/Fast - 3x or 4x ACOG
Fast - 1.5 - 5 x 20mm
Faster - 1-3 x 14mm
Fastest 1x Aimpoint (EOTech is just as fast, but the reticle on this one annoys me, and the battery life issue is a non-starter for me - the things gotta be on all the time, or it won't be on when you need it. )
All of that said, presently I am working towards setting up a long range AR, and that will probably have some heavy glass sitting on top. That's more of a toy for the 600 yards matches at my club, though and that rifle's "mission" doesn't involve being carried afield, or going to a carbine classes, or rolling around shooting from "urban prone" and what not. So long as a rifle has enough length to get the velocity up to reach out to 600 yards (18-20") I see nothing wrong with someone mounting a nightforce uber scope on that bad boy.