Francois Tigeot benchmarked his accounting work with blogbench, and posted a PDF with the results. Dmitrij D. Czarkoff made a simpler graph, which can be used to draw the conclusion: blogbench didn’t work well for estimating the impact of VFS accounting. If you want to try accounting yourself, put
vfs.accounting_enabled="1" in your /boot/loader.conf.
(The normal DragonFly mailarchive isn’t updating because it feeds from DragonFly NNTP, and that’s not updating, so I’m using Gmane for post links.)