I did some testing on this. Here are my results:
1. Prims made of Boxes or Cylinders receive the savings when grouped as convex hull. It halves their total cost. e.g. 40 boxes cost 20 prims.
2. Scripts negate this benefit. Scripted doors, etc.
3. Sphere, Torus and Tubes do not benefit from convex hull. Rings double land impact when linked as convex hull. Leave them as prim.
4. Sculpties may go up. Leave them as prim.
5. Boxes which have been hollowed cannot be walked through as convex hull.
6. You can group sculpties (set to prim) and boxes set to convex hull in one group. the items set to prim will count as one, and the convex hull set items will calculate accordingly. e.g. 10 items set to prim + 10 boxes set to convex hull = 15 prims total. Leave scripted items out of the group, even if they are set to prim.