IMHO there is (in most cases) no need for a cisco (or whatever brand you like ;-) router if you already have a viprion 2x00/4x00 in your datacenter. Specially if your trafficflows are so that 99% or so of them will pass the F5 anyway. The VS used for routing (forwarding ip) wont do any SNAT on the traffic, it will just shuffle the packets (that is packets that doesnt match any other better matching VS). Also when the F5 sits inline the need for SNAT will in many cases go away aswell (compared to when it sits on a stick).
Compare the following flows as example:
client -> switch -> F5 -> firewall -> server
vs
client -> switch -> ciscorouter -> F5 -> ciscorouter -> firewall -> server
and it can be even worse if the netadmin didnt think at all like:
client -> switch -> ciscorouter -> switch -> F5 -> switch -> ciscorouter -> firewall -> server
and so on ;-)