... meaning the exchangeable screen for lesser cameras also has zero effect for f2.8 lenses, but only for f2.5 or faster? I was never able to research how large the effect of the screen is for what aperture, and unfortunately you cannot try these in a shop.At 2.8 there is a slight benefit in real DoF, maybe just due to more definition between in and out of focus. However the viewfinder is slightly darker even at 2.8. Although they are called 'manual focus' screens I've found them to be beneficial only with very fast primes: f1.2 - 2, so they are more of a 'fast prime' screen really.
Chuck Westfall has stated, 'faster than f/4'. Personally, I don't see a lot of difference at f/2.8, but there is some. It's not really enough to change how I'd compose a shot, but at f/1.2 or f/1.8, the difference is very noticeable.