Victor, your benchmark is buggy (it writes one character at a time). You should apply the following patch:

$ diff -u bench_stringio_orig.py bench_stringio.py 
--- bench_stringio_orig.py	2012-08-11 12:02:16.528321958 +0200
+++ bench_stringio.py	2012-08-11 12:05:53.939536902 +0200
@@ -41,8 +41,8 @@
         ('bmp', '\u20ac' * k + '\n'),
         ('non-bmp', '\U0010ffff' * k + '\n'),
     ):
-        bench.bench_func('writer long lines %s' % charset, writer, n // k, text)
-        bench.bench_func('writer-reader long lines %s' % charset, writer_reader, n // k, text)
+        bench.bench_func('writer long lines %s' % charset, writer, n, [text])
+        bench.bench_func('writer-reader long lines %s' % charset, writer_reader, n, [text])
 
     for charset, text in (
         ('ascii', 'a' * (n // 10) + '\n'),
@@ -50,8 +50,8 @@
         ('bmp', '\u20ac' * (n // 10) + '\n'),
         ('non-bmp', '\U0010ffff' * (n // 10) + '\n'),
     ):
-        bench.bench_func('writer very long lines %s' % charset, writer, 10, text)
-        bench.bench_func('writer-reader very long lines %s' % charset, writer_reader, 10, text)
+        bench.bench_func('writer very long lines %s' % charset, writer, 100, [text])
+        bench.bench_func('writer-reader very long lines %s' % charset, writer_reader, 100, [text])
 
     data = 'abc\n' * n
     bench.bench_func('reader ascii', reader, data)