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.