Three options:

Upgrade to the latest stable version of TikiWiki

Pros

We can take advantage of the new TikiWiki features, such as Profiles (will allow us to create an "open source support" profile for TikiWiki that will be customized for the support website use case)
SUMO community will be used to all tools, including the wiki markup
Will pave the way for a healthy relationship with the TikiWiki community
- More visibility of our SUMO-specific features (e.g. the l10n dashboard) since they'll be available to other projects too
- We will become an actor instead of a consumer of TikiWiki -- our opportunity to give back to TikiWiki and demonstrate true Mozilla and open source values
- Opportunity to share our experiences and insights about secure and efficient coding to change TikiWiki developers' mindset

Cons

TikiWiki 4.x still has several major design flaws -- our best bet in the long term would be to rewrite some of these components to allow future scalability and extensibility
Making sure that everything still works on SUMO will mean extensive QA and likely several bug fixes aside from our initial list of patches to upstream.
TikiWiki and SUMO often has quite different priorities. TikiWiki wants to create the equivalent of a CMS "operating system" -- a solution that can do pretty much anything. Our focus is to produce a high quality solution designed to fit our needs.

Fork and leave TikiWiki behind altogether and develop our own solution

Pros

We could easily strip out 90% of the codebase
We could probably still "steal" new TikiWiki features if we wanted to by backporting
We could eventually rewrite major components and slowly get rid of our TikiWiki dependencies

Cons

We would still have to rewrite many of the components to ensure future scalability, but rather than working together with TikiWiki we would do it on our own
We could hardly call ourselves good open source citizens since we wouldn't give any of our fixes or feature additions back unless someone took our code and upstreamed it themselves

Pros

We could switch to a platform that focuses on scalability, performance, and extensibility, rather than a "Jack of all trades, master of none" platform

Cons

Unless we invested a crazy amount of time to make things feel unchanged, this would mean a radically different system for many of our community members' use cases:
- Editing articles would likely be done with a different wiki syntax
- Localization workflow would be different
- Forum software would be different
This would definitely disrupt and alienate many SUMO community members who typically aren't excited about changed workflows -- just getting people to contribute on SUMO rather than another system has proved to be very difficult in the past
Would essentially mean that all further SUMO development would stop for a good amount of time (probably a full quarter at best)
Not clear how much of a benefit this would be in the end -- depends heavily on the platform chosen
Would likely have tons of regressions and broken use cases we didn't consider