After using the Alpha 77 for a bit I realised that f/2.8 does make a significant difference to the EVF. I guess I missed this out from my test report. With an optical finder, you don't get any significant gain in brightness much past f/4, depends on the finder screen but the best ones like the A700/A900 get brighter up to f/2.8 (the grid screens) - however, they only get brighter up to f/2.8 because they are darker at f/4... etc. Modern consumer camera screens like the A580 are designed to be as bright as possible at apertures like f/3.5 to f/6.3 commonly found on zooms.
BUT EVF finders get brighter in direct proportion to the lens used, in low light. So, a 16-50mm used at 50mm at f/2.8 is passing twice as much light at the CZ 16-80mm at 50mm, and even more relative to the 18-55mm kit lens or the 16-105mm, or the 18-250mm (etc) which are generally f/5 to f/5.6 at 50mm. These lenses may be two stops less in light transmission.
That means the EVF has to apply two stops more gain in low light and you get a worse viewing image. Also, I realised that the new focusing module responds better to f/2.8 lenses.