Honestly, I'm pleased with the Sigma's price. My guess is that it will be a better performer than the 35mm L which is $500-600 more.
That would be a HUGE presumption IMO.
As presumptions go, I'd say it's just short of a racing certainty! The MTF charts indicate that the Sigma is a superior performer, which should be no surprise given its much more sophisticated design compared with the 14 year old Canon. We just have to hope that Sigma's build consistency is up to scratch.