Please not that depending on your switch SP engine and any modules, the bandwidth for a module is divided to a group of ports, for example ports 1,2,3,4, share the same bus, ports 5,6,7,8 share another bus, they are refered to as a blocking and non-blocking ports. You can provide us with little more detail about your switch.
If I were you and I'm in a hurry, I would first try to connect the FTP server's subnet directly to the SP engine, the ports on it are for sure non-blocking.
I think you should monitor the CPU and Memory utilization first.
