If they were reformed to 8x57 they would have to be shorter. There have been a couple of times over the last 50 years when 8x57 bullets/cartridges and cases have been hard to find. The 30-06 cases have always been available and because of their size they are ultimately suitable for reforming.
By the way, there have been a lot of 8mm bullets shot down 30-06 bores without incident. As mentioned though doing this in a valuable semi auto may not be prudent. Swaging the bullets down .015 by shooting them down a .308 bore will increase pressures with steel core ammo and not so much with lead core ammo.