I think some of the stuff about high rep ab stuff comes from the fact that your abs along with your calves are slow twitch muscle fibres. They are much harder to grow then any other muscle. They do grow some but genetics has alot to do with it as well. However, you can strengthen them alot but I think as far as just "getting a six pack" is concerned, you're better off spending your time nailing your diet to reduce fat and just throw in an ab exercise to your existing workout if it doesn't have it already. Twice a week is lots and I would try to get one rotation and one flexion exercice (not sure if thats the right word). You can also work on core stability (planks, rollouts, suitcase deads, etc) which have good tradeoffs to your big lifts.

IW