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Bert,
I have seen some of the images Scott has producing with that lens and it seems to have a similiar problem with bright stars that the Sigma 300 2.8 lens has (witness his M42 image). At the same time I think the Sigma has a bit more coma on one side but at the same time has less CA. I am not sure I would gain anything apart from going to the 300 EF non-IS.
Your EF lens is a cut above, even taking into account the larger pixel size of the 5DH. The level of aberations are very well controlled for f2.8.
The latest IS version seems to be a step back (its not just Jerry Lodriguss saying this either). I would be pretty upset shelling out $6-7 K only having to use the lens at f4 for astrophotography!
For the sort of work I do (comet searching) its important to have reasonably sharp images edge to edge whilst maintaining maximum possible aperture.
Terry
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