Since my last benchmark with XFS was kinda stupid (testing 512 MB of data on a box with 1GB RAM), I tested again, this time with 4GB of data.

"Sequential output" and "random delete" seem to be higher with an external logdev set (here: l_logdevhda5_size67108864)
In other places the external log seem to slow down operations (well, the logdev (hda5) *is* slower that the devices the tests were run on, but I somehow thought the journal would fit into RAM anyway. Hm, OTOH a journal written to RAM makes no sense, does it?
adjusting "-b size" from 4096 to 512 during mkfs(8) does not seem to change much, except for "sequential output" (+1MB/s) and 'sequential delete' (twice as much deletes/s)
adjusting the "-l size" to 4MB decreased 'random deletes' (with 64MB it's twice as fast)

The mountoptions do not seem to do much, but I really need to learn gnuplot(1) to generate nice graphs out of all these fine numbers....