Under Windows (64-bit Windows 7 on a  VirtualBox VM), the patch increases performance slightly but not as much as under Linux:

-> before patch:

C:\t\cpython>pc\VS9.0\amd64\python.exe -m timeit -s "import io; n=2000; d=[b'a'*
n,b'bb'*n,b'ccc'*n]*1000"  "s=io.BytesIO(); w=s.write"  "for x in d: w(x)"  "s.g
etvalue()"
10 loops, best of 3: 49.2 msec per loop

-> after patch:

C:\t\cpython>pc\VS9.0\amd64\python.exe -m timeit -s "import io; n=2000; d=[b'a'*
n,b'bb'*n,b'ccc'*n]*1000"  "s=io.BytesIO(); w=s.write"  "for x in d: w(x)"  "s.g
etvalue()"
10 loops, best of 3: 41.7 msec per loop


And the join() approach is 10x faster (!):

C:\t\cpython>pc\VS9.0\amd64\python.exe -m timeit -s "import io; n=2000; d=[b'a'*
n,b'bb'*n,b'ccc'*n]*1000"  "b''.join(d)"
100 loops, best of 3: 4.63 msec per loop

... which points to a much less optimized realloc() under Windows.