It's not a secret or anything, Sony says this on its website, there is no need to remove the mirror and run the tests. 30% of the incoming light is lost and used for focusing, so you get only 70% of the light, since every f stop is half (or double) the amount of light as the stop after (or before), so half of a stop is 25% of the light, so we're loosing a little more than half a stop with the mirror. But that is a price that you should be willing to pay for the faster, full time, always on, AF, otherwise you shouldn't buy an SLT. Also, I don't get where he got the 5% loss of IQ. Loosing light should (in theory) have nothing to do with loosing detail. I don't get the link. Taking pictures in very strong outdoor sunlight, loosing half a stop due to the mirror requires you to only adjust the shutter speed using the same ISO, why and how would the IQ drop? I have an A55, works just fine, if you want to know how not having the mirror in the way would perform better, read about the A580, which is exactly the same but an SLR.
Any time you put something in the way of the beam of light there is the potential for some detail to be lost due to refraction. The mirror is denser than air, and so the path of the light passing through the mirror is affected, however slightly that may be. So even the 70% of the light which hits the sensor would be subtly affected which could - in principle - affect detail.
This is all very theoretical though...I think the amount of effect is so tiny that it it is very hard to notice unless you're doing some strenuous pixel peeping. And there's no doubt that the SLT mirror, like a good UV filter, would have been designed specifically to minimize this interference anyway. The reality is also that light passes through multiple lens elements before it gets anywhere near the sensor so it has already been modified anyway, even in a traditional SLR. Oh, and don't forget that each little sensor node has something resembling a mini lens element over the top of it too, so that's another barrier the light has to pass through which could also potentially affect IQ. So while in principle yes, the mirror does affect IQ to some extent, I think sometimes some SLR purists forget that even in a traditional SLR there are number barriers which the beam of light has to pass through already. So the SLT mirror is simple "another" barrier rather than "the" barrier.
My personal opinion is that the IQ of the A580 has a minor edge over the A55, especially at higher ISOs, but I had to look long and hard at a lot of pictures to come to that conclusion so I don't think it's a difference most people would notice in everyday use.
