webproxy in high speed ADSL environment

If your router is connected to plenty of bandwidth, lets say 2 x 7,4Mb and we have hundred low usage clients connected with each 3Mb assigned, does a proxy server add to the speed of browsing or not?

The Pro of a web proxy will be that regularly requested web pages are probably available against the high speed network of the WISP (in my case the ´bottle neck´ is the capping of the client unit, so the 3Mb) so they can be available faster then taken them from the internet on ADSL or whatever slower WAN connections?

The Con would be that new sites, or sites that regularly change, are to be taken from the web anyway. Also, is the process of checking the availability of a page in the proxy and checking its time stamp and version number against the one on the internet not slowing down the process? Thus, if the connection to the internet is already fast, is there any gain to be achieved in using web proxy?

Some comments please…

rgds