IIS.net claims wincache will reduce filesystem I/O . php.net also recommends using wincache with IIS.
This does not appear to require a moodle plugin for wincache. The moodle plugin for wincache works with the 'User Cache API', not the File Cache (nice overview of different types of cache here).
I tried performance testing with and without wincache on php7.1/IIS7.5/M3.3 . Wincache was confirmed working using the status page that comes with the dll download, wincache2.php to monitor statistics.
I used jmeter to simulate visiting a couple courses with many activities and resources and clicking on a couple course resource links. Each page request used 'Retrieve All Embedded Resources' under the Advanced tab for HTTP request.
I did not see any significant difference in jmeter 'Throughput' and 'Received/KB sec'
I also tried the Moodle 'Caching->Test Performance' for 10k users and did not see any significant difference under the Application Cache->File cache section.
Seeing as iis.net and php.net recommend using wincache Im guessing my implementation or testing is done incorrectly. Any advice on how best to test performance? Or wincache configuration settings (I only enabled it)?