I have to admit that I have never given this topic much thought ... I do have the music (lead sheet/lyrics with chords/fake book copy/whatever) for a lot of the songs I perform sitting in front of me, and often I don't even look at it ... But after reflecting on this, I would have to agree that the best performances come from tunes that I don't have to be 'thinking' about the lyrics or chords... thanks ...

Terry ... I don't think it is the role of the orchestra player to 'interpret' his/her individual part ... The interpretation of the orchestration should come only from the conductor, and the audience should be able to feel that ... Also, the soloist wants no 'surprises' from the orchestra ... And, of course, the soloist should be soaring above the orchestra, letting us feel his/her interpretation of the piece, otherwise the soloist only becomes another orchestra member ... but many great classical artists give fabulous performances even if the music is in front of them ... I often wonder if they are actually reading it, or if it just a 'comfort' piece for them ...
I do agree however, that I do not think I've ever seen a 'pop' artist give a heartfelt rendition of a song lyric while reading it ...
t.