That’s already the way it works when it comes to finding the cell tile to search, so any cell outside the bounds of the search has no cost whatsoever - they aren’t looked up. I applied the same logic to the 3D case when I tried it.
It seems counter-intuitive to what you’d expect, but sorting cells into 3D never gave any benefit even with objects spread far and wide on the Z axis. BTW - objects are still filtered in 3D space, so while the first stage of searching takes place in 2D, the result is still the same as a 3D search 