First of all, Buehrle seems like the kind of guy who will get as far away from baseball fields as possible upon retiring.
Secondly, while "ace" is a subjective measure, I think you can at least say that he has not had the consistency you would like from an ace. In 13 full seasons, he has had:
An ERA+ of 130+ 3 times
An ERA+ of 115-129 6 times
An ERA+ of 100-114 4 times
An ERA+ below 100 1 time
I don't think of a guy with 38% of his seasons with an ERA+ below 115 as an ace. We all remember 2001, 2005 and 2007 really well but seem to gloss over 2003, 2006 and 2010.