I do not have a solution but maybe these experiments done with a 720 may help (I must thank our local reseller support here :).

We received an F720 some weeks ago (quad ethernet / FC9)

A 'systat 1' showed a really bad performance on the administrative Ethernet (100M) interface : the filer was inputting ~4MB/s when creating a 100MB file from a Sparc.

By moving the FC card from the slot 8 to the slot 2 and rebooting the (little:() beast we are now going to ~11MB/s .

Seems that netapp hardware layer is as poor as some PC cards builders :(

Maybe the trick will work for you : try to move your FC card from its current slot (8?) to another one.

Jean-Marie