I like the knockout idea...nothing worse than trying to knock out one size and the next size up breaks away too, so my vote is for no KO's
Something that I've only seen once is an FPE panel that had a KO for a duplex receptacle right on the can...thought that was a good idea.
Although I favour Siemens, the one thing I don't like is the buss bar sticking up....too easy for a bare ground to flip around and hit....I think FPE buss bar is good at preventing that. Maybe some plastic knock-out things over the buss bar that can be easily removed when installing breakers.
