Only being familar with the EDGE, which is similar performance most guys are shooting form 1-9 to 1-10 for the sierra. Mine is 1-10. Barrel profiles seem to start about sendero taper through to a heavy fluted 7-8 ? Just from what I read. I think most are using 30 " lengths plus break ?
With each step in the bush, you are a step closer.
The 338 lapua is a reasonabley effecient round meaning that the bore to powder ratio isin't that drastic. I'm not sure just how much velocity gain would be with a 30" over a 26" , I'm thinking maybe 100fps or their close. With a Sendero-#8 conture barrel that 4" will either add or cut alot of weight (maybe 3 lbs) and barrel stiffness , so basicaly what you have to look at is , Is the added weight and length worth the little extra velocity? if its a gun that not gonna be packed far or at all then in my oppinion I'd get the 30" with a 5" full diameter shank so it can be cut back later and rechambered as the throat wears , but if your gonna be packing the rifle up and down mountains then every little bit of weight you can cut helps and the animal will never be able to tell if the speed as a little slower.
Great question and I too would like to hear what others with first hand experience have to say.
I am having one built now, mine finished up at 27 .5" plus break. It is a 1/10 with a #8 contour. I too am going to shoot the 300's and if it needs a little more push we will pull it back off and Improve it. For now we decided to give it a go at standard .338 Lapua. Hoping to get 2850 from it. We will see.