Just a quick note about something I've discovered through debugging a recent WinCache issue:
If you change the wincache.scachesize value, you MUST shutdown all php-cgi.exe instances and manually delete the wincache_session_*.tmp file.
The wincache_session_*.tmp file will in the directory specified by seesion.save_path in the php.ini file.
An example session file name would look like: wincache_session_1_565779.tmp
If you don't delete this file, you will run into corruption in cross-process shared memory segments for the WinCache session handler. These will show up as 500 errors from your IIS server.