Of course, the costume you wear makes a HUGE difference as to the visual effect created for the audience. The costume is an integral part of the art. In that way, it definitely improves the ballet.
Other dancers' costumes might also improve my performance, because I see those costumes and interact with those dancers as they appear to me in costume. The effect can certainly be positive, but sometimes also negative. For example, it's harder to find your partner when she's in full makeup and hair in a bun and dressed exactly the same as 15 other dancers on the stage. I have to make sure that confusion doesn't show to the audience.
Finally, I would say that the lights are probably the biggest factor in producing "energy" for a performance. The theater is dark and the stage lights are on, and that just seems to rev everyone up to maximum potential. It's hard to maintain that same energy level for matinees, when we have a significant amount of natural light.[/i]