Your performance piece is important, but don’t forget about other things they’ll probably test you on.
When I auditioned for my BMus the performance piece was actually a pretty small part of the audition process. They were more concerned with my harmony and theory knowledge. There was a sight-reading test, an aural exam, and a written theory test. Playing wise they asked me to play two octave scales - I think they asked for modes too - definitely four note arpeggios. They were listening for how cleanly everything was executed, so if you play a five or six string make sure your muting is good.
Just try and stay relaxed, and good luck with the audition.