After reading this thread, I decided to experiment with this plugin too. Installed the plugin, made the changes I wanted in options, disabled the Kobo Glo default plugin, and restarted Calibre.
Like Joques, I was getting the error messages and no books were sent to the reader from my usual Calibre Library that is all DRM-free books. So I decided to try and send the free books that came on the Glo since I have never had access to them once I sideloaded books through Calibre. So I added them to Calibre, tried to send to the Glo, and same various errors, couldn't load them.
Since I really don't want the title up top, that was the end of my experiment. So I disconnected the Glo, uninstalled the Kobo extension plugin, enabled the default Kobo Glo plugin, and made sure all previous settings were unchanged. Calibre didn't prompt me to restart it for a change, so I just plugged the Glo back in, sent the books over without any errors or problems.
When I opened those new books on the Glo, they were indeed Kobofied... title on top was showing, covers were either missing altogether or wrong proportion (even though I had settings the same for the covers as the default plugin). Checked the device and the books were named .kepub.epub. The formatting on the these newly added books were just horrid as well, I figured it was a problem with the originals that I didn't check.
After checking the right plugins were enabled/disabled and options unchanged to get the default Kobo plugin working again, then restarting Calibre again, I check the books and the formatting is near perfect in them, the formatting must have gotten skewed in the transfer with the Kobo extension plugin. Transferred them again and the books were formatted correctly as they should have been and all covers stretched to fit as they should have been.
All I can suggest is keep playing with it... restart Calibre without device plugged in and try it again.
My experience with it was just odd. I'm using Calibre 0.9.29, and main memory on the device, I didn't transfer them to an SDcard.