"GNU hackers flatten Sun's professionals with they java.util.BitSet" - sounds not so bad. Some simple comparisons show that our BitSet runs roughly 24 % times faster than Sun's 1.6.0 implementation. Sun takes back on HashSet, however - this one is slower in GNU Classpath, more or less by the same percent. See the posted Advogato article for details.
I am not sure why our BitSet is faster. Deciding from OpenJDK, both implementation use the array of long's to keep the data. From the other side, the overall code is so different that it is difficult to say, who makes the benefit. The history of GNU BitSet spans over seven years (1998 - 2005) and have seen many contributions. The main authors of this class seem Jochen Hoenicke, Tom Tromey and Eric Blake.
I have checked near all java.util classes but the remaining differences seem for me too small to be considered seriously. I am, however, happy to discover that GNU Classpath is very far indeed from being universally worse than OpenJDK at any single line of code. The detailed 'class versus class' check may discover more interesting differences.