To avoid embarrassing yourself, at least try to get your facts right? Where did you find the ridiculous 50-60% figure?
The light loss is a rather negligible 1/3 *stop* loss of light only, a difference most people can't even tell with their naked eyes.
However, camera shake and mirror vibration is completely conspicuous to the naked eye. There is in-body image stabilisation of up to 4.5 stops with ANY LENS and the total elimination of mirror vibration shake at ANY SHUTTER SPEED. Whereas you wouldn't shoot handheld at 1/8th or 1/15th of a second previously, it'll end up being your favourite range of low light setting on a mirrorless setup (ask the NEX & m4/3 users who similarly do not have mirror limitations).
Its a massive leg up over a D600 or any other conventional DSLR.
DXO can never take those factors into consideration can it?