Re: seeking advice
Not a lot of difference in velocity between the big 338's unless you get into necking the 408 chey-tac case. Just depends on what you want to do with the rifle. You want primarily a bench gun or a hunting rifle to carry around all day? In a hunting weight rifle the 340 wby, 338 ultramag, 338-300 ultramag or edge, 338 lapua and lapua imp, and the 338-378 wby are all within 250 fps of each other. All will easily kill elk size game at 100+ yards. They go up the velocity scale in that order. I have all and all are very accurate if built right. If you want an over the counter rifle the remington 700 338 ultramag is a good rifle. Mine shoots 4 1/2" groups at 750 yards. The Weatherby 338-378 Accumark is the most powerful of the 338's in a hunting weight rifle and most shoot extremely well off the shelf. Both of those you can do under $1500. I am not a fan of the Sako's once they cut costs by eliminating the recoil lug on the action. No really good way to bed it for extreme accuracy.
If you go the custom route and it is a carry weight hunting rifle for long range hunting I would go for all the velocity I could get. Eliminating as much wind drift as possible is the most critical element of long range hunting and the quickest time of flight to target has the least wind drift. That is why when I get serious with long range hunting I always pick up my 338-378 weatherby with quite a bit more velocity than the standard Lapua or anything built off the ultramag case. I own and shoot them all. The 338-378 gets the most work when it really counts.
Predictions are difficult, especially when they involve the future