I've kinda steered clear of this thread for a while but will throw in my experiences as someone who has owned a few cars here including the latest which is an imported model from Japan not available officially in HK and which I believe is the most relevant.
When I went to replace my last car I actually spent a few months looking around until I was sure exactly what car I wanted. Once I did that I spent some time walking around dealer supermarkets (like HITEC in Kowloon Bay) and making a note of who had what. This helped hugely with price negotiations, etc as I knew what other dealers in the same venue had in stock. Now right from the start I got a cost breakdown, especially true with a trade in vehicle, and arranged an inspection with the AA. This cost breakdown stated deposit, car trade in value, registration fee, import tax, etc. In fact it was pretty thorough. What is important to note is that the cost displayed on the car included the import tax.
I would say that if the car you have bought displayed a price that did not include this tax then you may be half way to making a case, but of course this may also be affected by what else was written on the display. Certainly it's not fair to inform you of this cost after the deal has been done, you would certainly take the displayed price to be an on the road price minus certain government obligations such as registration transfer, road tax, etc. Go consumer council, see what they say first. They may themselves decide to take it further down a legal path.