Hi, I¬¥m a newbie and just ordered a NN3 MKII.
I own a Canon EOS 30D and the Canon EF-S 10-22 mm. wide angel lens, would there be a significant benefit to purchase a SIGMA 8 mm. fisheye making Spherical panoramas?
The 10-20mm Canon lens is no doubt a higher quality lens than the Sigma 8mm f3.5 and as Aussie Ninja pointed out your workflow would be lessened, and the resulting lesser workflow is great with moving things in a pano. I would further by saying even though there would be a slight degradation in quality that unless you seek to do large prints the output of the 8mm will still enable you to produce quality full screen panoramas for the internet.
In short and IMO if doing work primarily for the web - go for the 8mm if it is within budget.