You might try lowering SYNCER_MAXDELAY in sys/miscfs/syncfs/syncfs.h. I
don't know how much lower you'd need to drop it in order to see an
effect, or whether it'll solve your problems. However I should mention
that, although I might be senile and confused, I seem to remember Kirk
McKusick saying that he does not recommend soft updates on volumes which
are kept at near-capacity.

	What may be happening is that, because file removals or truncations
don't get sent to the disk immediately, the data is not immediately
reclaimed and the disk may appear full when it's not. Presumably it
should catch up within 30 seconds, which is the latency-limit before
things get flushed. Unless it's broken. 8-)
	-- Ethan