The compact command leaves no padding; it should have as an option to keep some, probably the same factor that what was already there.
SERVER-4018 adds an option to preserve padding on compaction (available in 2.6).
Initial sync in replication will result in fully packed documents, which is bad.
Repair will result in fully packed documents, which is bad.
Imports should have some options to be sensitive to having some padding, or some intelligence.
Padding factor should be easily viewable.
update docs when done