well i'm in exactly the same situation as you are .. sadly to notice it seems that people don't care much about these 'underdog threads' but just love to discuss about pixel peeping ISO stuff,.. instead.
i personally don't think that you will have a problem with the hoya pro1 cpl on a sigma 10-20 as it is an extra thin filter and i guess like all hoya filters made of satisfying quality.
so like nobody else seems to respond i guess it's all about on ourselves and somebody has to take the first step and try it out?!
i also would need the 77mm for a wide angle lens (i'll surely buy later) and need the step up ring for a 72mm standard lens and i think it would be just a waste of money for giving out so much money for filters when having that nice opportunity.. although i'm a little bit sceptical if this really works perfectly.