This is interesting. First, were a small country so our demand for cars isn't going to change international prices for cars. With regards to new cars, prices will fall by nearly same amount as fall in luxury tax, dealers are kind of in a perfectly competitive environment.
This will increase demand for new cars and decrease demand for the kind of old cars that depreciate (think regular 911). The reduction in price of used cars will be significant for newer cars than older ones as basically the reduction in price due t luxury tax removal will be less for cars that have significant depreciation rate (2006 Bentley continental will still cost around 70k at auction while the newer ones will be hit harder). A flat reduction of say 40k in the first year will be near zero after 10 years.
Collectible classics cars with a strong international rhd market will not experience any change in price as that price is internationally determined (look at prices at the Gosford lloyds auction).
So if I had a 2018 panamera and I thought luxury tax will disappear in a year and thought no ne else knows this, I'd sell now to the ordinary punter who hasn't calculated the effects of tax reduction. If I had a 2000 gt3 I wouldn't care too much.
Thing is, nice the reduction in tax is announced, prices will fall before the implementation due to rational expectations.
The interesting thing is that a reduction in luxury tax on new cars may even increase the price of say a GT3 RS 997 (2010). But not by much.