G'day Gregg. After a bit more research it appears we can go up to 330mm with 17" wheels. There are a number of 4 and 6 pot kits on eBay to suit VZ - search for Ksport or D2. These are probably made in China and not ADR compliant, which puts me off.
Next one I came across was a VF-spec upgrade kit with 322mm rotors, ebay item 181218009835, pictured below. Only twin piston but looks like a different design to the earlier calipers and the larger diam rotor will help.
A couple of places are selling an Alcon kit with 335x28 two piece rotors, 4 piston calipers, adapters, etc.