Someone has published first Sigma 10-20mm full size samples including the standard individual brick wall test now:
From the very few pictures I would expect the lens being a bit sharper (and it's wider on APS-C cameras) than my Sigma 12-24mm. Although there is some noticeable lack of sharpness in the corners at the wide end (10mm), I think this is a great lens for underwater shooting with an attractive price. And it works with old camera bodies (non EF-S) like I have. 
And close focus of 24cm doesn't leave something to be desired.
Distortion definitely seems to be much stronger than the Sigma 12-24mm. But who cares underwater. And distortion is easier to fix than lack of sharpness. 
Regards,
Julian
