A Some What One Arm Configuration on LTM

If you need to see clientIP, you could perform no-nat on F5 and insert a static route on the core switch in order to forward clientIP network back to F5. If your clients are all on the same network this might be the best option.  

I don't understand if your servers need to contact external resources via the default Gateway. If this is the case and the external resources are on the same network as your clients (eg. public/internet), with no-nat on F5 and only 1 NIC on the servers your options are either having F5 as a router for outboud traffic or performing asymmetric return/DSR.

For web applications, you can also perform a SNAT on BIG-IP and insert client IP in XFF HTTP header for the web servers to retireve.