It's okay if you know how to properly re-jet the carb for it.
Otherwise, it's a very bad idea to just run a straight pipe without re-jetting.
It will be quite loud, and you may end up with no real power improvement, and it might even be worse.
A straight pipe gives a very narrow rpm band of results, with the rest of the rpms being poorer. If you don't know how to set the length of the straight pipe for the specific rpm band you are tuning for, you may lose power everywhere, and gain nothing anywhere.
Except it will be real loud.
An unbaffled megaphone generally gives better tuning results, and will also be very loud. You can add a removable reverse-cone to damp down the noise a bit.