I have no experience with your suggested setup, but at least look into
as I have no windows systems.
having the NAS & SBS server on their OWN network adapter (1 Gbit),
not mixing the data with normal network traffic (i.e. use a wildly different IP address range for connection between the two, use a separate Gbit switch).
See that all equipment on that net allows for and is configured to use JUMBO frames (~10K ethernet packets) network interfaces of SBS server, NAS and SWITCH.
A network disk like you suggest would only give access the ISCSI disk to 1 system (it is just like a disk).
If you want to give more systems access to the storage then use a SMB/CIFS share on the storage appliance, that would allow for multiple systems access the info through a mapped drive.
So it depends on your exact requirements.
(Sharing a disk between two or more independent systems requires some kind of coordination schema, there are only few systems that implement such a system:
OpenVMS Clustering and GFS clustering, both depening on a distributed lock manager to "Get It Right", coordinated random access of multiple systems to one shared resource (like a multipath accessible disk).
