To continue some feedback..

NW6.5 SP3 as ISCSI Initiator
Intel Gigabit
single Xeon 3.0GHz / 800MHz FSB
Hyperthreading off.

Standard MTU=1500 CPU=25%
Jumbo MTU=4100 CPU=9%
Jumbo MTU=9000 CPU=7%

I had the Maximum Packet Receive Buffer Size set to 16400 in all tests.

Still working with Backline Support in NTS Utah to make my Jumbos faster
than standard frames - which may alter the above figures !

According to the packet scans things are tranferring okay, so now we are
going further up the stack...

Massimo - I mentioned the 11K windows size to them - see if I get an
explanation. ;-)

Stay tuned !