Terry the Sigma 300F2.8 EX HSM is a very good lens. I would try to get perfect focus at F2.8 as the depth of field is miniscule. I built a stepper focuser for my 300mm F2.8L and can see a difference in focus with 0.1mm linear movement in the focus ring at F2.8!
Here
http://www.iceinspace.com.au/forum/s...ad.php?t=11859
The problem with IS versions of any lens is if the 'park position' of the IS assembly is not exactly on the optic axis you have a problem with what looks like astigmatism but is due to misalignment.
Bert