There is a small 16bit penalty for the Pentium Pro

http://www.thg.ru/cpu/19970430/print.html

However, it doesn't measure as anything particularly huge. It was probably a little overblown back in the day because the Pentium Pro was a seriously expensive processor, and to have it not really beat a cheap Pentium at the same clock speed while running the everyday operating systems (win 95/98) meant that a you would be much better off getting a cheap pentium.

Then the Pentium II came out, Intel made a few small changes to improve mixed 16/32bit performance, and cranked clock speed into the stratosphere. That put a lid on the pentium's coffin.

MMX is a bit of a side show. A good boost in performance when used, but few applications actually used it until much later (and subsequently those applications are in the wrong era for the pentium mmx processor). MMX was must have been pretty limited, it took several instruction set extensions before intel knew what application developers wanted.