Filters most definitely can be used with the Sigma 8-16. Just like on its big brother the 12-24 f/4.5-5.6 DG EX HSM, the lens comes with a slide-on filter-thread holding ring. This ring is what the lens cap snaps into. Using that thread used for the lens cap you can use filters... on my Sigma 12-24 I've used Cokin Z-Pro filters for years.
Being an owner of both a Sigma 12-24 and a Tokina 11-16, I can say that the Tokina has better IQ than the Sigma 12-24... but the Sigma 12-24 @ 12mm on a film body or FF digital is the reason to own the lens. The Sigma 8-16 is a reworked version of the 12-24 for crops... 8mm UWA on a crop is about equivalent of 12mm on FF. And to the doubters all I gotta say is... "once you try it, you will want it". I will most definitely be getting an 8-16 this summer... to round out the family.
I'd NEVER buy the Canon 10-22... I think they are pointless. the 17-22mm that many 10-22 owners tout are really superfluous in such a lens. If you really think about it, its because of those 17-22mm that Canon couldn't make it a f/3.5 throughout the range, if not faster. The "telephoto" end of a UWA shouldn't be your priority in buying such a lens, and is quite oxymoronic if you think about it... yet every 10-22 owner lists it as a "benefit". I think the 10-22 buyers were just afraid to step away from OEM and are now justifying their decision with rubbish.
Agreed for these reasons plus the build. The Tokina 11-16mm is a great lens, and it does take filters. It has outstanding IQ and is built like an L. If I couldn't find the Tokina, I'd get the Sigma.