In regards to SSL, in the typical setup, you would create a Virtual IP listening on port 443 and add to it a pool of servers which are listening on their SSL port, lets say 50001, and that way the client's URI just displays https://VIP-HOST-NAME/ When switching from HTTP to HTTPS, you can just redirect to the SSL Virtual IP on the BIG-IP, if I'm understanding your situation properly.
Back to the topic of Web Dispatcher, I would say all the points you make are good reasons to outsource the job to a hardware load balancer. You will off load CPU cycles from the SAP server to the hardware load balancer, and you'll reduce hops and individual pieces to manage.
Hopefully this is somewhat helpful.
Nimbostratus