the CSS requires to see both side of a connection even if you don't need statefull failover.
There is no option for Direct Server Return (DSR).
You could have a different gateway on the server, so most of your traffic goes via the router, and then you need to use a client nat pool on the CSS so the server thinks the client is locally attached and respond to the CSS without going through the default gateway.
Only drawback is that the server will see connections coming from only 1 ip.
Gilles.