The test remaining is to serve up a page that has dynamic data on it and reacts to user input. Then put it on the internet to respond to other requests that fall outside the parameters you expect. Then do a few client calls to outside web servers, collate the data and feed it back differently.
Just as there is an upper finite limit as to how many people can be put in a car, there will be an upper limit as to what an arduino with an ethernet shield can do. It will never match Apache running on a powerful pc. So far I haven't seen any code that has both client and server functions included, but should be doable. The limited memory space available will require clever coding and thought to maximize what can be done with what is available.