I have 300blk barrels in 7.5", 8.3", 8.5", 9" and 10.5"
If I had a single 8.5" and was going to add something with an LVPO, I would probably look into a Wilson Combat 11.3" or something like that.
If I was only going to have one 300blk AR, it would be 8.5" or 9".
But if was going to have two, it would "longer and shorter" for the caliber. I've been eyeballing the new SIG 6.75" upper, and a WC 11.3" barrel or maybe even a BCM 12.5" barrel if they ever come back.
Sorta like if you only own one 5.56, it will probably be a 14.5/7" or 16". But if you own two 5.56's, then you would want something in the 10.3"-12.5" range and something in the 18"-20" range.
As for 9mm, I don't know, seems like the curve starts to flatten out in the 7"-9" range, especially when you click on the muzzle energy link. http://www.ballisticsbytheinch.com/9luger.html