Steven,

I've just looked at some 100% crops from DX images using a D5100 and that 28mm F/2. Nothing appears amiss, either focused at infinity or at the closest distance of 0.25 meters. In fact, I'm damn pleased with what I see. If I were shooting Kodachrome or Velvia or a new Ektachrome or techpan or something like that, on a tripod, with the mirror locked up, I might, might, give the nod to the f/2.8. But I shoot Tri-X so it's a non-starter. Plus I like that extra stop and the veiling at f/2. I've never been inclined to pixel peep and I'm less so every time I conduct one of these (for me) pointless little tests.

I still don't like that rattle but it doesn't appear to be causing any problems. For me. YMMV. Good luck and post if you take any action and what the outcome is.

Snap happy,

s-a