Moving from 4 to 8 GBit FC may help improving storage related tasks like Storage vMotion, however this depends on whether the storage system supports VAAI. On the other side, how often do you migrate the VM's to other datastores?
The 10GBit network will definitely improve vMotion tasks, but even without a 10GBit network you can improve vMotion tasks by setting up Multi NIC vMotion.
André