iSCSI - what's faster?

This depends entirely on your infrastructure.

Add a NIC to the server. Direct connect the DroboPro via iSCSI to the new NIC.

This defeats the purpose of iSCSI. Why not just get a DAS if you're going to do this? It would be faster.

Add a NIC to the server. Create a dedicated VLAN for the new NIC and Drobo.

This is fairly common, but you need to make sure your switch's backplane isn't being oversubscribed. If it is, you could have serious performance issues. If you can prioritize storage traffic across the backplane, do it.

Add a NIC to the server and attach it to a separate switch. DroboPro connects to the switch. Becomes a private network, similar to a VLAN.

This is also common. If you have concerns about performance or security on a shared switch, do this.