With some older cars Peter, enhancing their performance can make them safer and less stressfull to drive in modern traffic. The 500 wasn't considered a 'quick' car when it was new, so giving it just a bit more 'poke' can be adventageous. Beyond that, I can't give a sensible, inteligent answer.
In my case, having worked on 'the real McCoy' when I was a young mechanic made me WANT to build a replica '695'. People put bigger (and different) engines than 695cc into the 500, but as far as I am concerned, I set out to 'build an "Abarth 695SS". I shall go no bigger than '695' and tune it no more than Abarth would have done back in the day.