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-Preview
An error occurred while saving the commentVHosting Solution commented
The extension of plesk, must be able the customer to enable/disable the varnish cache for domain, so, if for magento is a nightmare, simply disable it from control panel and will works fine! :)