As far as I'm aware all JTD 16v and 20v waterpumps have plastic impellars. There was a rumour that the Brera/946 Spider JTDs are fitted with a metal impellar version, but I've yet to see one. I did ask my local dealer about this and apparently the same part number is used for all JTD 16v and 20v waterpumps.
The problem though with this waterpump is not the impellars. I'm not saying that they don't go wrong, just that in nearly all cases reported on here the problem is the bearings seizing inside the pump. This can either be sudden, leading to cambelt breakage, or the pumps begin to leak.
Approximate failure rate so far in the JTD waterpump thread is 57,000 miles or just over 4 years. I would strongly recommend you get your cambelt/tensioner/idle bearing/waterpump changed if your GT is over 4 years old. You can forget the Alfa recommended interval of 72,000 miles or 6 years - Russian Roulette I'm afraid.