Vignetting in wide-angle lenses

I was a bit surprised to see MR in the Voigtlander 15mm article describe a vignetting of 2 f-stops as "relatively low" for someone used to WA lenses on DSLRs. Even the APS-C wide-angle zooms (Canon, Sigma, Tamron and Tokina all) have significantly less vignetting, the worst being Sigma 10-20 with 1.4 f-stop at 10mm f/3.5 and the Tokina and Tamron both having less than 1 f-stop at resp. 12mm f/4 and 11mm f/4.5 (according to Photozone). The situation in Nikonland seems the same. The Olympus even has as little as .5 f-stop on a 22mm equivalent. Did I misinterpret something, or is there a happy surprise waiting for MR in wide-angle land?

-Lars