Powder-coating wheels is a cheap and nasty shortcut to a proper paint process IMO. You lose a lot of definition in the detail because the powder goes on so thick, and because its basically a layer of plastic it flakes off in large chunks if it gets damaged.
Whether you can diamond-cut really depends on how many times it's been done before. On the 20" wheels its very obvious when a wheel has been cut too many times as the fillets at the edge of the spokes start to disappear towards the centre. Once they have gone at the centre, the caps stick out from the wheel surface in a very awkward fashion.
Post a pic of one of the wheels and I can point out where this happens and whether yours have any more room for cutting
Personally, I'd just wet-paint them Avus silver
