So, do you need a CMS or something different? Writing a Web application that is data intensive would be outside the realm of a CMS, which is to store and retrieve content, and format it for display in a web browser. More or less, they are an easier way to separate content from style.
Yes, it uses flat files for storing content.
The nice thing about PmWiki, is that you can build any sort of markup you need, to include calling a php script to do the data collection/display; or using any sort of mashups you could think of.
This site is an example of PmWiki, with links to some other sites using it:
http://coreyreichle.tk