"Just wondering why are you thinking about doing that?
Owning a Mac is not about having the fastest and greatest, in fact for most of history owning a Mac = having a slower computer. It is all about the experience of using MacOS!"
thats not really true, apple has a history of releasing (or at least claiming) the worlds fastest desktops. and if you do 3d animation like me or video etc, YES it very much is about having a powerful computer.
and knowing whether you can upgrade the cpu could be handy to know for the future when chips come down in price.