I think that the only way to understand if all the indexed logs (or which part of them) are useful for you is to analyze informations in your logs: fields, messages, etc...
Analyzyng this, you can understand if there are events without useful informations and then exclude them before indexing using regexes.
An example to understand: if you need to know only accesses to windows servers, you need only few EventCodes (4624, 4625, 4634, etc...) so you could exclude events e.g. with EventCode=4688 (A new process has been created).
There are two methids to filter events: you can take only some interesting events and discard the others or discard only unuseful events and take all the other events.