Can you run an 1.5 kernel and see how softdep does? I'd like to know
   if something broke there, because it seems to be doing too much work
   now.


too much work?  it seems to be too little.  at least at unmount time,
from the softdep umounts listed:

 umount:       477.65 real         0.00 user         1.91 sys
 umount:       477.65 real         0.00 user         1.91 sys
 umount:        78.91 real         0.00 user         0.09 sys
 umount:       100.87 real         0.00 user         0.13 sys


in all of these, the cpu is well less than 1% active for the time
it takes to complete these rm ops and unmount the filesystem.


.mrg.