QUESTION: can I shorten barrels without compromising performance?

I know that is probably a daft question as all of these guns have an evolved barrel length that suits them (presumably).
But any ideas or general guidelines from anyone out there who has shortened any of these barrels would be great

Actually... IF a barrel length has been arrived at through evolved design to achieve the manufacturers perceived level of performance, what are the reasons for making them shorter? I can only think of:
  • design/aesthetics?
  • easier to use in 'tighter'spaces?