4 SNAT pools or a gigantic one?

I've used an in-line design with multiple HA pairs both internally and in internet facing environments on 1500 and 1600's without any issues even with large scale file transfers passing through. Not with connection counts like yours though and obviously it's something you'd want to test in some way; I can understand your caution.

 

 

Seems to me the current setup you have is ideal and there's no need to use an in-line design or SNAT. If the only way for traffic to get to and from the servers is via the F5 it's not required. Did I miss something?