I suppose read access is faster (but how much, given most of the processing time is spent inside templates?). Do you see a degradation in updating content?
the very strange thing: Inserting or updating content is a bit faster too (1 up to 2 seconds). But I don't have tested that a lot. My db has 300 MB, the largest tables are ezcontentobject_atribute (around 300.000 entries) and ezsearch_object_word_link (1.7 mio. entries). I am using mysql 4.1 and have turned the query cache on.
In the other case (if inserting is slowing down the system) I would also accept more indexes because over 90% of the queries are select queries.