(we had comment overlap above... Took a bit to type that comment ).
The memstore can have new or old data (back or forward dated), so it always needs to be considered (if that is what you meant).
I think I mentioned on the strip-compaction-jira... Every compaction that wants to remove delete markers has to consider the memstore and L0. Now it does seem that such compacation needs to consider all levels.
In the face of application defined timestamps that is indeed a bit tricky (and one area where we differ from LevelDB).
In the striped case each stripe is individually leveled, right? So we still get the benefit that a "major compaction" only has to consider this stripe (plus memstore and all L0) even if that includes all levels.