This brings into play another issue. How does one decide when he's going to "shoot the dot" or shoot off the top. Does that occur at 16 yards but not 17 yards?
I almost never shoot off the top. 99.9% of my handgun shooting is covering the intended POI with the dot/center tritium. XS recommends the "shoot off the top" sight picture for shooting at 25 yards and out. My experience however, with my handgun, is that it doesn't work. My rounds are still impacting in the center tritium at 25 yards. Like you, if my POA is the top edge of the dot, I'm hitting low. This is probably gun/ chambering/ammo dependent, but it's how it is with my G17. Getting out to
really far ranges is where I use the top-edge POA.
At 15 yards, I'm comfortable taking head shots on a standard IDPA target with the normal sight picture, as there's still plenty of 'head' visible to center the dot.
At 20 yards, the dot is completely covering the head. Hits can be made by indexing the IDPA target's shoulders between the sides of the dot and rear sight... but shots have to be taken slowly and deliberately. Accuracy and speed are suffering at this point.
At 25 yards, forget about it. I'm guessing about the dot's relation to the target's head. If I use the 'top edge' POA. I'm hitting the neck or upper body.
Big dots are NOT optimal for precise hits on small targets at distances where the dot completely covers the target. Pistol-training.com's "dot torture" drill is another example of this. At 3-4 yards the big dots work great. Once you start getting to 5+ yards away, the front sight starts completely obscuring those tiny little circles.
This is a limitation of the sight system that must be understood. Can I make better hits on a 20-yard IDPA target's head, or a 'Dot torture' drill @ 6 yards using a set of Heinie Straight-Eights? Yes. Do I think that's more important than the Big Dot's usefulness for putting effective rounds in the middle of torsos as fast as I can press and reset the trigger? No. YMMV, though.
I've seen some incredible shooters using XS Big Dots. I've also seen incredible shooters using traditional notch and post sights. None of them are guys that you want to be on the receiving end of in a gunfight. All of them have the talent, skill and training to place very fast, accurate hits into your boiler room with a handgun. Would changing up their sighting system alter this significantly? Probably not.
The important thing is to get a system you're comfortable with, and TRAIN WITH IT.