Made a test. Got my answer. It does sync amazingly well. It kept the identical files as they were and just added the missing files. Kudo BTSync!!
I have this 80Gb folder I'de like to sync across two computers in the house. I have already copied this folder to it's destination (Not with BTSync). So both are already cloned. If I sync them with BTSync now, will it overwrite or do the job all over?