That's a very high and very interesting number of users!!
I have no direct answer, but a few similar situations:
A- I had a wiki with 300 000 wiki pages (which were imported). At first (1.9.7), it would choke and eat up a lot of memory. SylvieG fixed up a few queries and tweaked the indexes and it has been fine since. These fixes were incorporated in 1.9.8
B- The mozilla support site (http://support.mozilla.com/) has been doing some work for high load & load balancing.
I think, with a bit of work, Tiki will be able to handle this. Can you coordinate with the TikiWiki community to get these fixes/enhancements in the main code base?
One thing that will need disabling/modifying is listing of all users in a drop-down menu (cause that would be 200 000 lines in the browser!). It should be replaced with a search username field (with AJAX to be fancy!). We have the same problem with our trackers over at http://dev.tikiwiki.org with 8700 users. It's bearable but unconfortable.