I think Nietzsche's Ubermensch is anyone would who can will his/her own standard of being and personal system of morality into power without relaying upon an externality as an absolute authority. Hence, 'God is dead'. It has more to do with our perceptual understanding of how the world works more so than any thing else. He used an example of a trapeze artist who performed his art for no other reason other than the fact that he loved doing it as an example of an Ubermensch.
Just a stray thought: I think Nietzsche would say that the desire 'to be better than average' is an example of slave morality rather than an example of an Ubermensch as you are essentially using 'the average of everyone' else in order to value your self as a human being. If you are are below average, you'll feel bad. If you are average, you will feel bad. If you are above average, you will feel good. Whereas an Ubermensch wouldn't rely upon the average in order to will his/her being into power. His/her feelings wouldn't be dependant upon the status of the average but would rely upon some internal standard of morality.