Well, the MTF from canon certainly looked promising, and PZ has pretty much confirmed it, razor sharp centres wide open and slightly better again stopped down a bit. I'd be a bit concerned about stop-down-focus-shift, but from f/2.8 to f/4 at anything normally framed (like 2+ metres away), that's going to be a non-issue.

Now the real dilemma, next time I want to take out the Nodal Ninja for some spherical panoramae at f/5.6, do I take the pancake or the Samyang?