This should go in the try at your own risk category.
Several years back, on a whim, I took an empty case that was in the cull bucket and using the taper crimp die dialed further and further down to see where the case would no longer positively headspace. I stopped at .460 and it was still stopping on the chamber lip every try. The thread is somewhere here, buried in noise. Now I personally would not crimp the mouth down that far in an active load, as that would compromise the bullets shape too much and yes there is such a thing as too much neck tension. Tried that also with some 200 XPB bullets utilizing two stab crimps, one in each of the driving band grooves. I would characterize the experience as more of a controlled explosion than ignition. Came away OK, but the 200 XPB's were clocking the traps at 2700 fps using a load that typically produced 2400 fps using Lil Gun! Turned heads up and down the firing line when I touched the first one off. I suppose you could experiment with reducing your loads or using a slower powder, but that's uncharted territory.
The point being, you can crimp the mouth diameter a lot further down than the Hornady recommended minimum of .474 or thereabouts, and not risk failing to headspace on the mouth. As for setting up an FCD to crimp right at the mouth, my experience has been that there is a tendency for the wall thickness to thin if you squeeze it a lot right at the mouth since it is only supported on one side of the transition as opposed to further down the case. You can not re-thicken thinned mouths. More in the cull bucket...
I have seen the effect of the gap in the collet fingers leaving distinct transition marks, but not strong enough to consider relying upon those three small ridges for a safe headspace. You're better off doing as WC recommends and creating a narrow bite line width for transferring the best, sharp, bite through the harder brass and into the softer bullet. If you do not thin the bite, the crimp is distributed too far over a range of wall and you wind up with a gradual hour glass transition. Bad for accuracy and bad for neck tension. The only caveat with the bite line width is you don't want it so sharp as to create a "tubing cutter" deep skive in the cases or risk them separating along that line. That one I haven't tested the limits of. I have already used up my 9 lives, thank you kindly
Taper crimping down in the cannelure groove or driving band groove provides adequate neck tension to satisfy the most picky accuracy buff and the loads are just as lethal at 2400 fps as 2600. The only plus side to striving for maximum safe velocity is a slight improvement in the maximum range that the XPBs will actually open up. There is the added benefit of less barrel jump which ultimately slows down has fast you can reacquire the target. The 200 XPB loads I use for whitetails is tame by some standards, but they're a pleasure to shoot and recover from, not to mention the distance I encounter whitetails at, in the Northern Minnesota woods, don't require long flat trajectories.