Shorter scopes have faster focal ratios and more distortion of the image at the edges of the field of view. The longer you can make the focal ratio, the easier it is to get a flat distortion free field of view over the magnification range. The ATACR has superior optical performance compared to the NX8, mostly due to the appochromatic glass and the longer focal ratio giving a nice flat, sharp field of view compared to the NX8. So there is a very valid reason the ATACR is longer than the NX8.
I went with the Spuhr QD option.
I’m currently running both my NX8 and my ATACR in Geiselle mounts. I really like them and they are hell bent for strong, and the RTZ has been incredible. My ATACR goes from my 5.56 to my .308. All I do is make my pre noted adjustments, and it’s always PERFECT. I have my 2.5-10 in a NF Unimount. Very pleased with it as well. Much more affordable, and very solid as well. It MIGHT, and I’m not positive, but might not be as good for a LPV 1-8, 1-6, due to it being a 20moa can’t. I know this can make LPV’s look funny because it makes them not truly flat on your eye. Never tried it due to having the Geiselle.
The cant could throw off the perceived profile, but unless you’re super switched-on I doubt anyone would notice let alone care. You’re GTG, brother. I do like the Geissele mounts though. Very slick, and it earns extra cool points for aesthetics and perceived no-BS ruggedness. I can see why Garand Thumb fans would slap their momma for one. I like what Spuhr offers, albeit I hate paying for them. But I do prefer them, and for that reason alone I suppose the purchases are justified. The adjustable QDP fits the 1-8x ATACR beautifully, looks almost like a single unit. My 35mm Spuhr houses the new Leupold Mark 5hd MPVO. Another example of beautifully paired components. Regardless, we keep good company, my friend.
It actually will cause distortion, per Nightforce. Flat is best.
NF told me the ATACR is fine in a 20 moa mount. Only the NX8 is recommended to be used in a 0 moa mount.
It’s very easy to compare. A 20 moa mount requires you to dial POI down approximately 5.8 mils. On my ATACR I cannot tell any difference whatsoever when I do this. The image stays exactly the same at 1x magnification.
I was told it does it on any FFP optic, but some more than others. I am sure it does it on the ATACR, it’s just academic in nature instead of noticeable.
Well, the atacr does have a big booty. All that glass counts for something. 
I should probably trade my atacr 1-8 in for another 4-16x42. It spends 90% of the time on 8x, and the rest on 3-4x.
I’d like to also add that Spuhr has a taller 34mm mount (1.89" or something) while everyone else has the standard 1.54".
I’ve submitted feedback to Geissele for a taller 34mm mount in the 1.93/2.04 flavor so hopefully that is happening soon.