I wun say that FF is more demanding (other than the fact lens have to performance very well at the border in order to avoid softness around the corner), i would rather say that it is the 22MP which is more demanding...
Exactly, the details this level of sensor can capture will magnified flaws on the lens design.
Also to add my comment on Sigma by C J Goh, that was quite a surprise the amount of OT that generated. I was one that was once victim of some senseless flaming. But I think C J Goh may had taken it a bit too personally. The comment on Sigma are rationale and objective, and by no means pompous, or elitist.
Canon L lens don't get the price tag and support from such wide consumer without it's quality, and so does Sigma. But sigma is like a very good Honda (Proton is too much of an extreme), while Canon L is a Lexus. Bear in mind there are some pretty lousy Lexus along their line-ups, and so does L lens. Measure by quality/price ratio, Sigma is probably going to win, so does Honda. But there is a market for Canon L, and there is a market for Lexus.