Benefits of "monitoring" are way high. So you can't call it "overhead". But for this, you would be analyzing periodically. Now, *thats* a real overhead. Why analyze when you don't have to? Thats what monitoring does for you..
In any case, you won't see any noticeable performance difference. Statistics are in memory and are flushed to disk periodically - every 15mts.
PS : If you know for a fact that an object is being heavily updated already, it obviates the need for monitoring those objects. If you aren't sure, that's when monitoring helps you out.