1:9 is the most common twist rate put on factory ARs. I wouldn't bother spending more on a faster twist barrel unless you are planning on handloading for it using VLDs or you are buying a specialized varminter gun. Most of the bulk (cheap) .223 ammo is going to be in the 55gr range anyhow.
FWIW, my 1:9 Bushmaster HBAR seems to shoot everything well across the board in factory ammo.
For the 77 and 80 VLD's you need at least an 8" twist.
The best highpower shooter that I personally know uses kriegers in 1:7. How he explained it to me is that a 1:8 will stabilize a VLD very well in a bolt gun because of the extra bit of velocity a bolt gun offers and ok in an AR, but further downrange the 1:7 in an AR makes a difference. I don't know if what he says holds true or not, but I can't shoot well enough to tell the difference like he can.