Which cache is working with cwp apache using suPHP?
Since CWP is working with suPHP handler you can use Varnish only.

Nginx as revers proxy will also help as it will be used for all static file and will provide better stability and lower number of request for apache.

Varnish will work with suPHP
Memcached will work with suPHP
opcache will NOT work with suPHP
xcache will NOT work with suPHP
apc cache will NOT work with suPHP