I have the 700 VTR (1:12" twist, 22" barrel including brake, so about 20.5"). The 168gr. bullets shoot great to 750 yds. for me, the 178gr. bullets shot ok until about 250 yds. Then at 450 yds. I was getting about 8" groups (unacceptable!). My 750 yd. groups with the 168gr. are about 2.5 - 3" on a windless day lol.
In my experience, the 178gr. is too heavy out of a 20" 1:12" twist barrel for long range shooting (I was only getting about 2450 fps out of my barrel with the 178gr.). You need either more velocity or a faster twist (basically a longer barrel or faster twist). 168gr. is the "perfect" bullet weight for a standard .308 Win.
P.S. Remember this is an opinion based on personal experience, not necessarily a fact written in stone lol