The 338-378 wby is still the top dawg out there. I currently own and have built through the years most all the big 338's and all the popular ones discussed on the sight. The 338-378 wby is the fastest other than the 338-416 rigby imp. which is about the same. Barrel life is exceptional with it. My first one is many years old and well over a thousand rounds through it. It still shoots 1-1/2" groups at a quarter mile. I have killed everything I ever shot at with it at some extreme ranges. All those animals never argued about what brass I was using. I guess when it comes right down to it performance is everything. You can argue freebore, brass, bla, bla, bla but the performance is tops. Bottom line the 338-378 wby outperforms the rest. One big 338 is just as accurate as the other all else being equal.
Now, having said that there are many other factors to consider. For that performance brass is expensive and you need a mk 5 action or a custom. You will burn about 15-20 more grains of powder per shot. The huge 378 case is long and the shorter imp lapua case allows you to seat the bullets out further to the lands and gets within 50 or so fps of the big 338-378. Now that wby brass has gone through the roof and the lapua imp uses 15 grains less powder per shot at nearly the same velocity it is now the more popular choice for a custom build. That, and now you can get a 338 ultramag that is only 150 fps slower in an over the counter remington that uses cheap remington brass. Those are the two main reasons the 338-378 wby is not as popular among custom builds anymore. As far as an over the counter rifle the 338-378 wby accumark is still the top rifle out there in a big 338. It may cost a little more but the quality of that rifle is worth it. About as close to a custom as you can get.
Bottom line is in a custom now the benefits of the lapua imp overshadow the 50 or so fps advantage of the 338-378. In a factory rifle you can get a 338 ultramag rem 700 for $600-$700 that comes within 100 or so fps of the lapua imp. My over the counter rem 700 in 338 ultramag shotts 4" groups at 750 yards and will take any elk to well beyond 1000 yards. In this economy when people start looking hard at a rifle that will efficiently do the job the 338 ultramag makes a lot of sense. But if a guy wants the extra power and willing to pay the money the wby accumark 338-378 is there.
In real world tests I have done many of there is only about 250 fps difference between the 340 wby and the 338-378 wby. All the other big 338's fit between these and all will take an elk beyond 1000 yards which is way further than anybody but a small few can shoot. So many people seriously ask themselves , why pay the extra money and keep paying everytime they shoot.
