#2 - usually the journal should be pretty quick to allocate - I've not experienced this problem directly myself.
I'll add some extra bits to the bottom of the post with your notes.
The Mongo team seem somewhat reluctant to implement it.
Compression could have drawbacks on documents that get updated frequently. But it will be extremely useful on documents that get created and rarely/never change, coincidentally what I mostly have.
It would also greatly help if keys are compressed or indexed in some way since it could be done transparently.
You may recall the Mongo team being reluctant to make the database function well in single server setups, but they did address that with journalling.
Also, I'm working on Snappy compression with Mongo (it's already used for the journal), however it's not currently stable and work is sporadic due to my startup.