For BC its math if you keep environmental at standard.
I have a ton of respect for George Gardner and his team. But everyone has their specialty. But I will error on the side of the specialist in the field of expertise. GAP rifles bullets Berger and AB testing. More twist unless extreme has no practical negative.
Maybe I am mistaken but as far as I can tell the cartridge design itself is entirely irrelevant to needed barrel twist rate outside of its average velocity for a given bullet and that really has far less effect than air density changes. Does not matter if its a 6.5 WSM, RSAUM, SS, TSS, S, 6.5/06 etc...... If they are all driving them the same basic vel then they will all use the same basic twist rate. For most all the 6.5s you see the 1:8 is standard for the longer high bullets. Especially if its for shooting into critters at longer ranges there seems to be a mounting pile of anecdotal evidence that more spin gives better terminal ballistic effects.
Do not forget the effect temperature has on air density as its much like elevation. The colder the air temp the denser the air the more twist you need to achieve the same stability factor.
Say you have caliber and bullet that has stability factor 1.3 @ 8.6 twist rate @ temp of 100 degree air temp. In the winter at the same elevation with temps in the high 20s low 30s you would need a full extra inch twist to achieve the same stability factor.
If you go for the optimal stability factor in the METRO/ICAO/standard atmosphere which say would be 1.5 at standard 59f temp sea level 29.92 hg density basically shooting in most every extreme of weather regardless of elevation is going to ensure that bullet will always be stable and in most conditions will give its maximum BC as well. Basically you want your twist rate to ensure stability for the highest air density you will ever find yourself shooting that combo in. That would be the lowest altitude coldest temp.
Lija has a article he us to have up on his site or maybe still does that talks about the relationship..
Edited to add: Here it is:
http://riflebarrels.com/effects-of-altitude-and-temperature-on-rifling-twist/
I agree 100% with the last sentence of the last paragraph:
"As can be seen though, it is safer to be conservative when it comes to twist, and err towards the faster rate."
Depending where you live or shoot at that 8.6 may have all the twist you need to stabilize any of the 140s to 1.5 especially if you do not shoot in the winter but are at higher elevation there maybe nothing to worry about.
