4 SNAT pools or a gigantic one?

I'm afraid I've not the experience to offer a sensible opinion regarding the timeout.

 

 

I don't think there would be any performance difference difference between 'in-line' as you describe it and using an SNAT (and presumably having the real servers on a network not directly attached to the F5).

 

 

There are other options but it's hard to know what to recommend without understanding your infrastructure and the traffic profile. Regardless, do you need SNAT at all? If this is internet sourced traffic can't you just route as appropriate when required and put your servers wherever you want?