As a proud 30-378 owner I'll share some of my observations.
1. Accuracy is what it is, my mk5 shot sub moa, if this is a sako it's likely a custom so I would have reasonable expectations. The round is capable of great accuracy having been a long range wildcat for years.
2. I would not shoot factory ammo, you can reload for around $1 a piece with a good bullet if you have the brass.
Your just going to have and try the gun to find out but I would not hesitate because that is a great action.
I had numerous long range loads for hunting and target using bullets from the 230 Berger to the 180 TTSX and shot over 1200 rds through mine out to,ranges of 2000 yds before having it re-baralled to a 338.
My best loads used RETUMBO at slightly less than max loads and bullets seated to magazine length, the why freebore makes OAL kind of a non issue
My son and I killed at least one of every big game animal in NM with that gun including an Orxy at 650 yds........
I actually have some brass and dies for sale in the classifieds right now if you end up with the rifle.
FS-30-378 Weatherby Brass & RCBS dies