I'm skeptical such lens will perform well. But I'm willing to be proven wrong. When a lens is designed to fit a ruler instead of deliver optical performance....things usually don't work out.
Normally, you're right, and that's probably why canon's never made a pancake before (afaik).
But comparing the MTF of the pancake, 50/1.8 and 50/1.4, the pancake looks to be clearly winning.
Whether that's because of a rushed and/or lower-quality design of the 50s, or because it's easier to design slower lenses, or just 20 years-newer coatings or what, i don't know.
To be fairer we should compare the Pancake to the 50/2.5 Macro, anyone got the MTFs of that?