Would this work or even help with performance in any way?
I suggest you don't use this on the default files. Especially prototype.js, editor.js and global.js
you are better off setting up a proper cache/expires block in .htaccess to force the browser to cache the data for a longer time period. There are a few posts by Euan T. and myself with specific examples you can search for
It really doesn't make that much difference unless you have lots of large JS files really. As pavemen mentioned, it's best to set up proper browser caching.
I have proper caching setup
Minifying JS won't do much to help then I'm afraid. That isn't to day you can't, but you won't see any drastic speed increases.