there was a local garage that appeared to have a business in simply selling the metal impellar pumps. However this company went under last year.
Metal impellar pumps definately exist/existed, however im not sure they were VAG/dealer stock held parts.
I had my water pump and cambelt changed by a stealer and at the time I was in a similar situation. I asked the steamer to source and fit a metal one but they said couldn't source one and it wouldn't be covered by them if it went wrong. So ended up with the plastic one they recommended.
They also said that yes the plastic one could fail but wouldn't cause as much damage as a metal one failing.
That doesn't mean to say I've heard of a metal one failing.
i fitted the metal one myself, it was a 3rd party product,
the metal impellor will not physically break, if anythings to go on a waterpump barring the plastic version it'd be the seal or phospor bushing, which if seized would strip your cambelt anyways,
i wouldnt personally worry about it, a lot of water pumps are plastic impellored, a big factor that causes an impellor is the incorrect anti freeze mix in the winter, in which the coolant can freeze and causes damage once the engine is fired up, = bang dead waterpump