Chrome bundles a PDF renderer. (You won't find it in the open Chromium sources; it's by proprietary license from a third party.)

http://blog.chromium.org/2010/06/bringing-improved-pdf-suppo...

It's already sandboxed and pretty fast and light-weight, so I'm not sure the Chromium developers would put much work into a replacement.

It looks like Chromium doesn't have it but Google Chrome has a built in PDF viewer.

Right. Chrome uses a binary PDF reader plugin, there are no sources available. Open source projects like Chromium and Firefox require an open source reader. I think MuPDF would be a good choice.

Does MuPDF have any speed/performance/etc benefits over the plugin in Chrome? Are there any reasons to spend the time/effort to switched other then one is open source?

I wasn't talking about Chrome switching. MuPDF is an open source option for open source projects.