IMHO, Sierra has a tendency to state that faster twists are required, but I think the more customary twists may still be adequate at the higher velocities available from the longer barrels.
If I am correct, Sierra's tendency is likely based on their inability to control/predict what length barrel and/or which chambering is going to be in use by the customer. After all, this bullet will fit a range of chamberings including the 6.5 Grendel as well as the 6.5-284, and even the 6.5 SAUM.
So capabilities vary broadly, and I think something like my own 28" 1:8" .260 Rem just may be effective with the 150SMK.
Anyhoo, I'll be watching the preliminary efforts; and then, I'll probably try a box's worth for pressure and stability testing at various charge levels. It may even turn out that I'll be quite able to avoid max/near max charges.
In fact, I suspect that this bullet will highlight the performance of H-4831 in the .260, thus reducing the insane demand on H-4350.
Greg