I have to admit I am not following you at all. We are looking the same idea from totally different perspectives.
The original post was "looking for optimized way to sync only existing leads".
In the last update you mentioned "I dont getLeads before updating. I'm calling syncLead and using marketoCookie in ParamsSyncLead for lead identification."
When I read "looking for optimized way to sync only existing leads" I interpret as "update attributes of existing leads, do not create any new lead". I would use getMultipleLeads or getLeadChanges, uptade all relevant attributes and then syncMultipleLeads based on lead key IDNUM.