We as an Aluminium foundry, have found that using serated chills have greatly improved the quality of the castings. The reason for its can be put into two categories.
1. Increased surface area for chilling.
2. it helps reduce blow off (where the chill blows part of the casting away from the face of the chill) the typical defect looks like a large depression in the metal at the metal chill interface. I think that having a flat surface sitting tight againnst the metal means no venting is possible. We have found that the serrations eliminate the chills being blown off, and help reduce shrinkage.
To add to this the latest thinking is that fins should be used attached to the casting, this eliminates the need for chills. Though im still doubtful as the ability to control directional solidification.
i would say take a chill, take an grinder and take make a lattice network across the chill and try it.