gstripe performance oddity

i tried sending this to freebsd-geom but got no response, so i'll try  
a higher traffic superset list...

i have a bit of a weird issue, which i suspect is a configuration  
problem, and was looking for a little advice.  i have an LSI JBOD box  
with a bunch of SAS drives that i would like to gstripe together.   
each drive individually seems to be able to do about 80 MB/sec  
streaming write, and doing parallel dd's gives me the 160 MB/sec i  
would expect.  if i gstripe them together with a 256k stripe width, i  
only see 80 MB/sec, though.

if, however, i newfs/mount it as ufs and then dd myself a big file,  
that gets me about 120-130 MB/sec.

why does mounting matter?

- j