My .260 was purchased used and has a 1:9 twist. My MGM barrel in 6.5 Bullberry IMP (aka 6.5x30-30AI) is a 1:8, which is the rate I prefer in anything 6.5mm.
The previous owner of the .260 had developed a wonderful load with the 120 grain BT, and I saw no need to experiment any farther as I also got a ton of those as component bullets with the barrel.
But the barrels I have had with a 1:8 would digest everything from the stubby 85 grain Sierra HP through the streamlined 140 A-Max and Berger VLDs -- and group all with extreme precision. Within reason, you can't overstabilize a bullet, but you can certainly understabilize one. And that's why the 1:8 is my preferred twist in anything 6.5mm.