You can use the head command with a boolean expression, and it will allow the streaming search to continue until the boolean expression returns false. This can be a neat trick especially when you can have it entirely in the streaming portion of the search, because you can get far fewer events off disk sometimes.
In this particular case though, the eventstats command is going to run through the entire search and so when the head command terminates it, it'll be too late to avoid getting all the events off disk.
Anyway, it would look more like this:
... | streamstats count as thisRowCount | eventstats count as totalCount | head thisRowCount<totalCount/10