Should I be expecting better sharpness from 55-250?

I snapped this picture of a Red Shouldered Hawk the other day - there's a pair of them that frequently perch on the corner of a house down the street from me and I managed to get fairly close to them without them flying away.  I was hoping for sharper pics and am a little disappointed to be honest.  Is this the best I can expect from this decidedly consumer level lens?  *Picture has been cropped

Here's a full size version for anyone interested (remember, it has been cropped)

http://www.deviantconstrictors.com/images/HighQuality/IMG_0350lg.JPG

Taken with the 60D, manually selected the focus point to be on the Hawk's eye.  Shutter speed was 1/320 which, with IS engaged, should have been plenty fast at 250mm.

I've kicked around the idea of getting the 70-300L, but I don't use the telephoto range often enough to justify the price tag.

Thanks for any advice/insight...