I believe you're right, in that controller ownership is per vDisk. So if ownership changes, it would be for all LUNs that are part of that vDisk.
Last week, just to verify, I did successfully test path failover on a SAS-attached MSA1040.
I have two ESXi hosts, each with two SAS ports. there is one LUN configured. Each SAS port is connected to a different MSA controller, so the LUN has two paths from each host - one Active, and one Active(I/O). I pulled one SAS cable out of host 1, and storage connectivity stayed available with VMs still running on both hosts. I then plugged that SAS cable back in, ensured both paths showed active again, and uplugged the other SAS cable on that same host. Again, storage connectivity stayed available.
If it works with SAS, I'm not sure why it wouldn't work the same with iSCSI. Unfortunately I don't have access to an iSCSI MSA to test with.
