Yes it makes it louder. Yes it's stupid.
The loudness depends on the size of the hole.
It shouldn't harm anything in theory, except the exaust system itself, it will rust faster and I'm not sure how it'll be looked up during inspection.
There is virtualy no performance gain from this at all, unless you downright chop half the exaust off...in which case you're kinda hosed cause you lost your backpressure...
In other words don't do it. It's idiotic.