what would a happen, as far as performance and sound, if you just totally cut off the exaust of a gs500e. just wondering

First of all, it would sound horrible.  Have you ever heard a car without a muffler?  It's just noise- no music.  

Along with having flames shooting out of the front of your engine block, performance would suffer as I'm pretty sure you need SOME backpressure.

under a certain length header (I'm not sure what that measurement is) there is a possibility of getting cold air into the motor and that could have damaging effects on the valves...or so I've heard.

It would sound like crap and it would run like it sounded.

I've tried it before with my two part Yosh pipes.  
Yeah it sounds stupid. instead of a growl or "thump thump thump thump". it sounds like "bloop bloop bloop bloop".

Didn't even bother trying to ride it like that. Theoretically an open header of the right lenght (probably the stock length) will greatly increase top end flow. However mid and off idle where the back pressure is most useful will be hurt, might even be unrideable on a parrallel twin...

Its noise not music like he said...Sounds raw instead of tough but it was fairly driveable at mid and low speed...(dont remember much it was an accident that I rode it like that). The cold air issue can be addressed 2 ways...use a rag and stuff it in when you turn it off, or use a vane type close mechanism that the truckers have on their pipes.
Cool.
Srinath.