You should be able to set the a specific MTU independently from the options offered in the device drivers properties via netsh as described here:
http://www.richard-slater.co.uk/archives/2009/10/23/change-your-mtu-under-vista-or-windows-7/
I tested this and it seems to work.
You're probably aware of it but remember to also adjust the vSwitch MTU if you're doing this.