The one copy of the 24 f/2.8 f/2 AIS that I tested did not seem dull, but while it resolved fairly well in the center, edges were far from sharp at infinity, and did not quite recover when stopped down to f/8.
Attached examples resized frames with D7100 handheld at f/2, f/2.8 and f/8, then 100% edge crops at f/2, f/2.8 and f/8 and a center crop at f/2.8 and f/8.
[Edit: Oops- this was f/2 version as in the thread title, not the f/2.8 version. Sorry for the confusion, now corrected and it is should be clear from the rest of the text too.]