Article http://complement.sourceforge.net/compare.pdf present comparison of strings implementations in STLport (trunk) and libstdc++. This tests also can help to answer on questions: COW or non-COW strings implementation better? What allocator is better? Under what conditions? This comparison is done on the base of STLport strings, ropes and libstdc++ strings. I expect that this results will help to make decision between STL implementations as well proper choice of strings implementation. And, of cause, it will help in future development of STLport.
Performance test suites and results are in STLport SVN repository: SVN https://stlport.svn.sourceforge.net/svnroot/stlport/trunk/perf