In general, if you create a new query and observe events on it, we'll retrieve everything in the query from the server. So in your example, we'll re-retrieve the first 30 items when you do the limit 60 query.
The one exception is if you have an observer with no query at all at a location, then any queries you do at that location or underneath it will not retrieve anything from the server, since we know for sure that we have all of the data.