Firstly, what is a filtered browser? These are custom-built web browsers that push all content through a content filter, rather than directly out to the internet.
This is similar to what curbi does, except it only covers content from that browser. Since it is very much an 'app world' now, locking down a single browser has very little effect on what a phone or tablet can do.
For a filtered browser to be effective you also have to switch off Safari (in Restrictions) and prevent all app downloads, since there are many alternative browsers available on the App Store.
A very large number of apps also contain built-in browsers themselves now. This is because Apple has made it extremely easy for any app developer to drop in a web page viewer into any app with a few clicks.
For these reasons we think that filtered browsers are of no real use on Apple devices. Instead, we have gone with the approach of filtering all content that goes to the device, regardless of the app being used.