I strongly suspect that SAN running interop mode 3 would be able to support a connection to a switch port running in NPIV mode since that switch port should appear as a device port. Additionally, after looking at the Cisco documentation on interop mode 3 and interop mode 2, all of the changes appear to be fabric config type changes to enable E-port (or ISL type connectivity) between the two different switches. Since the NPIV enabled port should appear as a end device, those config changes shouldn't matter. The only caveat I can see if if NPIV cannot be enabled on the switch/port running in interop mode 3 but it appears you are already looking at testing that bit. Sorry but this is a long-winded way of saying I think the test would be valid.