Summary of unchanged Python (2.4 .. 2.7):
 * Mac: subprocess is 25 .. 50 times SLOWER than os.popen
 * Solaris : subprocess is 13 times SLOWER than os.popen
 * Windows XP : subprocess is 1.5 times FASTER than os.popen
 * Linux : (results are very close)

With a different buffer size:
 * Solaris : Popen(bufsize=-1) is FASTER than os.popen()
 * Mac : Popen(bufsize=1) and Popen(bufsize=8192) are a little bit 
slower than os.popen(), but much FASTER than Popen(bufsize=0)

Notes:
 - PyFile_SetBufSize(bufsize) does nothing if bufsize < 0: keep system 
default (buffer of BUFSIZE bytes)
 - On Ubuntu Gutsy, system default (BUFSIZ) is 8192 bytes