A lot depends on temperature as well as elevation. In warmer weather (typical of Texas) a "slow" twist will stabilize bullets that the same twist won't in truly cold weather--a potential problem here in western Montana, though it's somewhat counteracted by the higher elevation.
The rule-of-thumb is that every 5000 feet of additional elevation acts about like a 1" faster twist. So if you have a 1-9 twist 6.5mm barrel and don't shoot at any lower than 4000 feet in elevation, it will stabilize even with 140 Berger VLD's even at -20 Fahrenheit. The stabilization won't be enough to optimize ballistic coefficient, but the bullets will remain point on and probably be pretty accurate.
The 4000 feet is my situation, by the way. The town we live in is at 3900 feet, right along the Missouri River in the bottom of the valley, so every place else around here is even higher, including the two ranges I shoot at. As a result a 1-9 twist works for any 6.5mm round with 140 VLD's.
Even at sea level a 1-9 should stabilize (but not optimize) a 140 VLD down to -20 degrees. The reason many prefer a 1-8 twist isn't because it won't keep a 140 VLD point-on, or prevent them from shooting accurately, is that BC isn't optimized until the Sg is 1.5 or more.
Sg is a number assigned to a bullet's stability in a certain twist and conditions, with 1.0 the minimum needed for stabilization. But BC will increase up to a 1.5 SG, which is why many prefer a 1-8 twist, not because a 1-9 twist won't stabilize long 140-grain bullets. But it will result in a 1.5 Sg at 7000 feet above sea level, at a typical fall hunting temperature of 35 degrees.
Bullet stabilization isn't a simple either/or situation. It's affected not just by rifling twist, but by temperature, elevation and whether or not you want the highest BC the bullet's capable of.
