Also, if you could play 6 Highways is likely that:
1. you can win by 3-pile as Davio said.
2. you can buy more than 1 province per turn and come back on pure green, without attacking (for instance, buying any $5 with +Buy should easily give you one more province and possibly more instead of "just" removing an opponent Province, getting him a Gold or Duchy -or Alt VP- to compensate).
3. your opponent also can buy more than 1 province per turn and 3-pile at any point.
I think it is rare that Highway->Saboteur is the best choice, it may be a desperate move, or something to compensate if you have extra actions but no +Buy nor gainer. UNLESS you can KC or at least TR the Saboteur, that can be a devastating turn and you can do it and then 3-pile for the win, getting a massive VP swing.
On the flip side, if you can pull it off fast, 2 Highways -> Saboteur will mess with your opponent engine by killing its 5s early, instead of its Provinces late. If your opponent has a good engine, it will quickly recover from a Province lost much more than a semi-built engine from losing its key $5 card.