Dat rear... element though. 
But I'm wondering, could this shape have exacerbated the vignetting issues?
P.S. there's a typo in the hood description on page 1... Should be "barrel-shaped" (even if I have no idea how it really looks). :-)
P.P.S. you mentioned the 24/1.8 being "nothing special"; have you happened to test it already?
P.P.P.S. FWIW I wouldn't count the 16/2.8 RF among the pancakes... It's almost double the size of this 28.
But I'm wondering, could this shape have exacerbated the vignetting issues?
P.S. there's a typo in the hood description on page 1... Should be "barrel-shaped" (even if I have no idea how it really looks). :-)
P.P.S. you mentioned the 24/1.8 being "nothing special"; have you happened to test it already?
P.P.P.S. FWIW I wouldn't count the 16/2.8 RF among the pancakes... It's almost double the size of this 28.