I always flash my lights or wave to other dubbers, mainly Mark 1 or Mark 2 GTI's, and some mark 3's. However 4 and 5's dont really know what they are driving tbh. I dont always get a response but sometimes I do.
I find that when I am in the golf and I wave to scirocco owners they dont tend to respond either as they dont like golf or golf owners, or at least thats the vibe I have got from scirocco owners.
I dont wave to aircooled as I know I am not going to get a response, if they waved to me I would be only too happy to respond