Reading this thread reminded me of the test I made a couple of years ago with a DX11 video card and I was able to get 26fps for 1,000,000 (that's one million) cubes! I don't think it was vanilla Irrlicht, but it gives an idea of what is possible.
See my post in the thread here for more details:
http://irrlicht.sourceforge.net/phpBB2/viewtopic.php?t=34448&start=19