Oh right. I didn't think about the extra time that signal propagation through a big ugly ROM would take.

I my comment earlier about EDA tools being able to "simplify" the ROM into the die was probably a paraphrasing of what I was told about a tool. The tool (or workflow) allowed a designer to go from a working prototype microcode ROM to a hardcoded gate circuit (presumably by the same sort of process that one can go through to simplify a logic statement that is in disjunctive-normal form).

This was information passed on to me second hand from quite a few years back now so I probably should've tried to look up some primary sources before writing my comment! Oh well.