to answer your original question, yes, it will be softer at 2.8, most lenses are like that... "f8 and be there" is how it's phrased?
here is how i understand it: when you hit that optimal f8 or whatever it is for a particular lens, things will be sharper because of the increased resolution, not because there is a bigger area that's in perfect focus.
however, more of the image will be in an acceptable focus range because of the increased depth of field, which i believe is called the "circle of confusion":
Depth of Field Calculator