This is a situation where the physical disk configuration may make a real difference. If the journal and the data files are on the same disk, a write-heavy load will be continually seeking between them. If they're on different disks, there's a lot less seeking, and that may reduce the performance penalty quite a bit (at the cost of extra hardware).

And of course, if you're on SSDs, this is all a non-issue, but that also still comes at a premium.

I didn't but I would be interested.

FWIW, here's the FAQ on performance with durability enabled:

How's performance?

Read performance should be the same. Write performance should be very good but there is some overhead over the non-durable version as the journal files must be written. If you find a case where there is a large difference in performance between running with and without --dur, please let us know so we can tune it. Additionally, some performancing tuning enhancements in this area are already queued for v1.8.1 and beyond.