I definately think rob's right. I'd rather want to hear a performance just slighty under tempo, where you can manage to express the music, than hearing a performance, where you would just be struggling to survive the pieces.
When you're doing it a bit slower you also have a better chance to bring out more of the fine details and textueres of the music that you might not notice when playing it in the real tempo. Allow yourself to enjoy it, and when you finally bring the tempo up a notch you'll be much aware of what's happening in the music.
Sometimes I've had too little time to bring a new piece up to the tempo I really want before a performance, so I "played safe" and did it a bit slower. One or two after I normally feel the difference, I feel more secure, and I have no problems at all playing the piece much faster.
Good luck with the festival!