Interesting comments. These lenses are used on a bolex 16mm. I'll try out the light meter trick, however, this camera doesn't offer that much light out the viewfinder.
I've also just completed a test, and the result with only the 10 mm lens, and then the 6 mm lens did not have much difference in light loss (at least not by a stop, anyway). So, it would appear that focus would now be my main question - and if you factor in Chris' comment of 'sucking more into a lens', perhaps the lens remains at it's base lens focus (in this case 10 mm) and as an addition, the frame is provided a wider angle. So, given that the 10 mm lens contains the preferred depth of field, adding a 6mm adaptor will offer more viewing to the scene. Seems like a bonus to me...