A WSGI application will be fine this is mostly a backend question and data processing question, in my opinion as that is where more architectural parts come into play. I would look into using Celery ( http://celeryproject.org/ ) for your work distribution and backend scaling. Twisted would be a good choice, but it appears you already have that portion written for use as a WSGI application so I would just extend it with Celery.
I do not know the scope of your project but I would design it with Celery in mind.
I would have my frontend endpoints be the WSGI (because you already have that written) and write the backend to be distributed via messages. Then you would have a pool of backend nodes that would pull messages off of the Celery queue and complete the required work. It would look sort of like:
Apache -> WSGI Containers -> Celery Message Queue -> Celery Workers.
The apache nodes would be behind a load balancer of some kind. This would be a fairly simple architecture to scale and is, if done correctly, fairly reliable. Code for failure in a system like this and you will be fine.