I think it would probably be pretty tasty and might make it better. It already has some chocolate flavor due to the dark grains but more won't hurt. I don't know about the mint but what the hell, might be pretty good but I just wouldn't over do it. I think you can add just about anything to this and will be pretty decent.
I have a hard time keeping this one around. As a matter of fact I'm down to the last 6 pack. I will probably brew this again soon however it will be 10 gal batch. One of my SWMBO's friends is a big stout fan and she is funny now because she compares this one to all of the commercial stouts available around here and she would rather drink this. Others have told me the same thing.
"Look, let me explain something to you. I'm not Mr. Lebowski. You're Mr. Lebowski. I'm the Dude. So that's what you call me. That or His Dudeness... Duder... or El Duderino, if, you know, you're not into the whole brevity thing." -The Dude