Yeah, i pretty much understand how server queries work, that's why i was asking if the sync would require said query or not.

Unfortunately, i don't think there's any workaround to it, so as i said the only way to do a partial sync would be to find out ourselves which contacts are registered before we attempt to sync them. (or to blindly try our luck with a few selected contacts, as you said, which would sort of defeat the original purpose of not giving them more numbers than they already have)

The drawback of this is that (i suppose) it won't show automatically contacts who register in whatsapp sometime after we'd done the sync, so we'd have to wait for a message from said people to add them manually, or sync them manually once we found out they had registered.

As i see it, this kind of feature is useless for regular users as it is. But I'm glad you decided to take the time to provide a solution for those paranoid people (no offense intended) who can't bear with terms of service.