I shoot my 16" Grendel at much farther distances than I ever would have, had I not been challenged to shoot it out to 1200yds last summer. I do live out West at much higher altitude than Florida, so that helps, but the fact that I have a little carbine says a lot for the potential of a 20" gun at sea level in Florida's thick air. This past weekend, I was spotting for an invitational precision rifle match, and shot my 16" Grendel to 1500yds with factory 123gr SST afterwards, but I ran out of elevation and reticle, since I don't have an MOA cant base on it. However, my impacts were very predictable. It would simply be a matter of using a 20-30 MOA cant, plus a scope with enough elevation to get it done.
I also have a .260 Rem AR10 with a 22" barrel. Compared to my 16" Grendel, I gain 200yds of supersonic performance with the 123gr target bullets. If I had a 22" Grendel, it would be a difference of 90-125yds. My .260 Rem upper receiver group weighs more than my scoped 16" Grendel, and cost about 3x as much.
The real value of both of these cartridges is in the projectiles and their efficiency. In all the testing I have done with the .260 gassers, the 130gr VLD's and Norma's are the balancing point in terms of BC and speed, with the 139-142gr tipping over on the slower, heavier, less flat side.
I obviously have both, but if I were forced to pick one all-round, it would be the Grendel for a number of reasons, the most important being that it actually has very affordable match ammo that's actually meant to work in an AR15/gas gun, whereas the .260 Rem is exclusively a hand-loader's affair for me.
If I wanted a rifle specifically geared to shooting 1 mile, I would normally knee jerk to the .338 LM or a 7 SAUM, but I want to experiment with some tight twist smaller calibers in the meantime and see what happens. I think Todd Hodnett is onto something with shorter bull barrels and a tight twist for ELR. Spinning the bullet unusually fast makes sense for a possible solution for passing through the transonic flight envelope, and he has been doing it with 16" bull .308's using the 175gr SMK, as well as 20" bull .338 LM TRG's.