As said, a 26" barrel really is a waste of a lot of powder. Most rifles reviewed shooting factory ammo do not meet the 3000fps 250gr goal of the orig Lapua design. 2800 to 2900fps seems to happen more often then higher.
That is not a significant difference over the 340Wby. The 338 RUM would certainly get things moving without any fuss, so why am I excited...less expensive components.
The 338 cal does not have a lot of options in heavy high BC match quality bullets. With interest in the Lapua, lots of new toys will emerge. See post on new Hornady bullets.
These will work just fine in my 338 Mystic which has a 36" barrel. Plus the improved Lapua case is quite a dandy in a long barrel. A bit larger then the 338 Mystic.
As to accuracy, all of these cases are equally accurate, or can be made so. However, the more powder you burn per bang, the harder it is to tune for max accuracy.
Jerry