the effort involved in maintaining the one's I've seen must cause tension in all the wrong places, resulting in a weaker performance.
I completely disagree. When a person is feeling a certain emotion whilst playing music, it's quite normal and natural for his body language and facial expressions to reflect that. In my opinion the tension and weaker performance would occur from trying to maintain a facial expression (poker face) that is contrary to the emotion one is feeling.
When I see a person performing a very emotional piece where, for example, the music is supposed to express some extreme emotion, such as heartbreak or rage, I find it
inappropriate to see a blank facial expression. It makes me think the performer is not truly lost in the music.
Richter and Hamelin are two examples of performers that show very little animation of the face and body language, and I think this has an effect on how I view their performances. When listening to the audio only, I often think "wow, that was intense". But when seeing the video with the reserved mannerisms I tend to start to reconsider and wonder if the stunning performance I heard was just contrived, as the performer doesn't appear to be that 'into it'.
On the other hand, it can also go the other way. Where, for example, the music has a slight happy/upbeat feel to it, and the performer has a facial expression that looks like he just won the 100 million dollar lottery.