Of course, a virtualized Guest will have a performance penalty.

With that phrase I meant that, following this guide, you can make use of the Virtualization Extensions from the Cortex-A7 which powers the Raspberry Pi 2.

This is pretty useful for running multiple isolated services (such as a media server and an ownCloud instance), doing some kernel hacking, or testing a variety of ARMv7 distributions.

It's probably not meant about performance, but more along the lines of "now you can use every feature it has".

The overhead can be very small, but the overhead is besides the point. The point is that it for example allows you to run things in parallel that you otherwise would need to reboot to run. E.g. testing a different OS or distribution.