I also read somewhere (possible here in fact) that the older chrome finish lenses were sharper then their newer black finish lenses. However, as described above there's lots of factors that go into perceived sharpness. The main reason why I went with a chrome 80mm lens (I do now own a black version however) is because my friend I mentioned above said that the chrome lenses were built more sturdy specifically the shutter. According to him the shutters in the newer black lenses were made with cheaper materials as a cost saving measure. As a result, if any of these shutters were to go bad, rather then service the shutter as was the case with the chrome lenses, Mamiya would simply replace the shutter at a substantial cost. Anyway, I wasn't surprised that my 80mm chrome lens needed some TLC. I picked it up today, apparently there is a spring involved with the shutter cocking mechanism that needed replacement as well as general cleaning. So hopefully it will work, I'll take it out this weekend and see. Besides, I still haven't found a lens hood for the newer black lenses.
By the way, I also have the 55mm lens and yes, at f/4.5 it's excruciatingly slow. I tried to shoot with it hand held indoors with 400 speed film and it was near impossible, so it's an outdoor lens unless I can use a tripod and cable release. with it indoors.