1.5" group is less than typical. My AR Grendel does better than that. My mini Howa bolt gun, much better. I'll love to try to group some rounds - let me know if you want to go out and play with some loads.Is your Grendel on a AR platform or is it a bolt action rifle?
I don't currently load for the 6.5 Grendel, but a good friend of mine recently purchased a mini Howa 1500 bolt gun. Initial results using Hornady Black ammunition, either with the 123 gr ELDM or the 123 gr BTHP bullets, or Hornady Custom ammunition with the 123 gr SST bullet, have not been particularly good (best 3-shot group has been 1 1/2" at 100 yds). So, I'm guessing he and I will soon start with hand loading. I'm pretty confident that I can find something that does better than that!
From what I have read, Hornady uses a ball powder in their factory loads. I suspect that they use the same ball powder in all of them. And, my impression is that my friend's barrel probably doesn't like that particular powder.