As of late 2015, Virus Bulletin has been running regular comparative web filter tests (the VBWeb tests).
These tests, the focus of which is currently network-based solutions (web gateways), measure the ability of participating products to prevent malicious traffic such as malicious executables and drive-by downloads from infecting an organization's network.
Like all Virus Bulletin tests, the VBWeb tests are comparative, allowing developers and (potential) customers to find out how products perform in comparison with their peers. Consultancy options are also available.
For more information, please contact Martijn Grooten firstname.lastname@example.org.
The video below is from a VB2012 presentation that explains the difficulties faced when testing the accuracy of web filters.