Would you mind to try Nginx caching in Plesk Onyx 17.8 Preview?
This would be very helpful to get your experienced feedback here http://www.surveygizmo.com/s3/4165754/Plesk-17-8-PreviewBen commented
Your Website is much faster with varnish. Varnish is not about heavy load sites like Apache+Nginx its all about fast webpages. Its much faster than php based alternatives. All major sites using varnish or spdy. I don´t understand why Sergey closed this thread? The advandage of varnish is very easy to understand.