Mongo is already a performance leader, but it would be great to push this farther via an option for storage entirely within ramspace like an Oracle In-Memory Database or MySQL memory table (aka. heap table)

This would also need to turn off the disk-space pre-allocation; with storage in RAM you would not need any pre-allocation since re-allocating additional space would be an almost instant task.

I have created a hack for this that works but even with nssize=2 and smallfiles set the default disk pre-allocation makes my setup rather limited in use.
http://www.phpmoadmin.com/mongodb-in-memory-database