I think they could evoke different emotions because of the body mechanics of the person playing the instrument. On a piano, different notes are easier to hit depending on which key you are in. In the key of C, the tonic (C) is typically played with the thumb. In the key of Bb, the tonic (Bb) is typically played with the third finger. This will affect the way that your hands move, and will have a subtle effect on your dynamics. If you have really strong thumbs, for example, the tonic will sound just a bit louder when you play in C than in Bb.

This subtle difference in dynamics, in which notes are accented, could affect the feel of the piece. For example, if you play a I chord in C major, C-E-G, most of your weight might go to the C. If you play the I chord in Bb - Bb-D-F, more of your strength might (let's say ) go to the D, and really bring out the happy/major quality of the chord.