I would guess 1-8.5 or a 1-9 twist would do the trick - especially with the big 30 cal magnums (300NM, 300RUM, 30 Nosler, etc). Even shooting the 200gr bullet with a BC over .7 is crazy. My 300NM would send those down range at 3200fps easy.
Id be pretty shocked if it required an 8. I’ll bet it’s very similar in length to the berger. Until someone chimes in with its length or test fire data, it’s all a guess now.
I've run the 230 Bergers just fine in 10 twist 300WM's and 300RUM's, I know apples to oranges since the Sierra could easily be longer and require more RPM but it should give an idea. I certainly don't see it needing faster than a 9 however.