alter mongostat to include 'flushes/s' parameter from db.serverStatus(). Including the ms timing would be cool, but I think trying to keep the output compact is important, so just putting the # of flushes/sec in there is fine. Generally it would only be 0 or 1. I have a hard time believing the db can do > 1 fsync() per second. But it's still very useful.
The idea is to provide a column that shows why performance degrades during an fsync(). So when using mongostat and the throughput reduces one can see that "oh, we did a flush, thats why..."