Yep I play guitar and performance anxiety is something I struggle with too. I find that when playing guitar, the key to overcoming performance anxiety has more to do with manipulation of the mindset........than it is to do with manipulating your playing style or muscles. I'm not sure if this is how it works in stuttering though. As you said, you've had a different experience with modifying speech (something that I'm trying to grasp at the moment). In the past I've tried to use the same method for my speech. I get over performance anxiety when playing guitar, by not caring about messing up and by having fun playing. So I've tried to use that in speech. I've noticed that when I've been enthusiastic about talking, the stutter becomes less.