I have opened the mufflers up on several of these 590's and 600's.
The first one I did was the same way you did yours. To be honest it had the most obnoxious exhaust tone I think I have ever heard out of a saw. Like I said earlier a "poppy" idle and extremely "raspy" at WOT, to the point it was hard to tell if it would 4 stroke with light pressure in the cut.
I feel they are so loud opening the muffler up this way is because the exhaust comes out of the cylinder and its more or less a straight shoot up and out through the deflector.
So now this is how I open them up and they sound really good IMO.
Its a lot more work but 100% worth it.
(Sorry guys, I couldn't edit this post with pictures)