Every object is stored in the grid as a blob whether it is a simply object or a collection. It is not possible to retrieve just a single item from within a serialized collection. The client cache can alleviate the overhead involved in retrieving the entire dictionary. You can also consider breaking up items in a dictionary so they are stored as individual objects within a named cache, which offers similar semantics as a dictionary.
Another approach is to use an API feature called "single method invocation" (SMI). It allows you to run the your own custom logic on the ScaleOut host where your dictionary object resides. You can run code on the server to extract and return just the portion of the dictionary that you are interested in. This API requires ScaleOut Analytics Server license. Please let us know if it is something you would like to try, and we can send you a temporary license key and a sample code that illustrates how to call an SMI method.
