The only reason to get a 1:8 twIst is because you can't find a 1;7 barrel. There is little disadvantage to a fast twist, but plenty of disadvantages to a slow twist. The most obvious is that the slower twist won't produce stability in longer bullets. Now I'm speaking to you as someone who lives at sea level and shoots in the cold, so by definition I need a faster twist to stabilize the same bullet that a fellow living in a warmer part of the country, and shooting at higher altitudes uses successfully. But mono-metal hunting bullets and VLD target bullets tend to be longer then traditional lead core semi-spitzer flat base bullets of the same weight, so expect them to do better with more twist. Conversely, there is no loss of accuracy with the use of short bullets from fast twist barrels.
But the fast twist has other advantages. The long range shooter using a fast twist barrel will not experience the loss of stability as his bullet crosses into the transonic velocity range as does the shooter with a slow or "optimum" twist barrel. The hunter will observe deeper penetration on big game from the same bullet if his twist is faster, although this is easier to see when long bullets are used. The reason for the deeper penetration is that the bullet precesses (yaws) for a less time when it impacts resistance denser than air than it would if it were spinning slower. The less time that the bullet is in yaw, the less resistance there is to its forward progression, so the deeper it penetrates.
So what are the disadvantages of a fast twist barrel? I've been a fan of fast twist for quite some time now, and I've only seen 3 disadvantages of a very fast twist. One is that if you're shooting bullets with very thin jackets, the fast twist might cause them to fail prior to hitting the target, or once the target is hit, the expansion is more violent and penetration is dramatically reduced. This was my experience when shooting Hornady SX and Sierra Blitz from a 1:7 .22 center fire. Therefore choosing the appropriately constructed bullet is more important if you have a fast twist barrel.
The second disadvantage of the very fast twist barrel is for the cast bullet shooter. With a very fast twist, the velocity of your cast bullet will have to be dramatically reduced. If you've been shooting cast in the 1600-1800 fps range, expect to reduce that velocity to 1200 fps to prevent severe leading and poor accuracy. I discovered this when attempting to shoot cast bullets from my 1:8 .308, so now I simply save my cast bullet shooting for my 1;10 and 1:12 twist .30s.
Lastly, a bullet fired from the fast twist barrel is effected more by spin drift. I don't consider this a significant problem myself, as spin drift exists with any bullet fired from a rifled barrel, and it only becomes an issue at extended ranges. But spin drift is a constant, and is therefore easily compensated for.
IMHO slower twist will allow higher MV's b/c there is less spike in pressure behind the bullet compared to fast twist barrel.
