Right, the result of yet more testing.
To answer my own question 3. "testing the performance of a network drive" you can use a program called PerformanceTest v6.1 which enables you to test the performance of network drives and compare them to previously saved baseline for comparison.
Download
Credits
To answer my own question 4. To set the MTU under Vista you can use a command prompt command called netsh
Credits
netsh interface ipv4 show subinterfaces to show current MTU
netsh interface ipv4 set subinterface "Local Area Connection" mtu=7418 store=persistent to set a new MTU of 7418
Cool - Now I have my router set to 7418, my Vista PC and I know my NAS has a drop down that supports it.
So I set the NAS and rebooted and hay presto I can ping the NAS, SSH to the NAS but can't bring up the web interface or browse the windows shares held on it.
So knock my PC back to 1500 (netsh interface ipv4 set subinterface "Local Area Connection" mtu=1500 store=persistent) and it all instantly springs into life. So I do an # ifconfig eth0 on my NAS to make sure it is set for 7418 and sure enough it says MTU is 7420 (two more than it web interface)
With all three devices set to 7420 they can ping each other with a fat packet (ping -l 7420 under windows ping -s 7420 under linux/freebsd) except that the windows PC can not ping the nas with a fat packet only with one of 1500 but the NAS can ping the PC, so I think that the NAS is lying and not capable of running with JUMBO frames even though it says it should so I will be giving the Qnap guy's an ear bending on their forum.
Anyway, in summary Jumbo frames are supported and quite easy to implement and when I get my NAS playing ball I will be able to confirm the increase in speed.
Cheers and hope this has given some people food for thought.