I can't lose the extender until I can afford a real 600mm and the Sherpa to carry it
But I do not think it is the extender as I have closely looked at distant shots at large magnification and they are sharp. The problem is with subjects about 15 feet distant.
Are you certain that shots at 100ft are that sharp?
As stated by a previous poster, you might not notice the difference at 100ft because depth of field is much greater.
The general formula for DOF is (when the distance to the subject is much larger than the focal length):
DOF = 2 * c * a * (d*d) / (f*f)
Where c is the confusion circle, a is the aperture number, d is the distance to the subject and f the focal length.
If we take f=0.560m, d=30m, a=8 and c=5E-6m we have a DOF of 23cms
If we take d=5m, we have a DOF of 0.6cm (36 times less)
Thus something which is in focus at 100ft might be out of focus at 15ft, and unpleasant things can happen if you lens is misaligned or not properly calibrated.
(If you want more details on calculating DOF, look
here for example).