I bought a 1991 Lexus LS400 about 12 months ago. The vehicle is due for some new tyres. The manufacturer's specification is 205/65 ZR 15 and accords with the vehicle's touring speed potential. Clearly that potential is way beyond any legal speed limit within Australia! At least one individual in the tyre trade has suggested that, provided the replacement tyres meet the minimum ``load'' rating requirement, there is no need to fit a Z-rated tyre.
The load rating is critical to maintain. There is no legal requirement to fit a particular speed-rating tyre to any car. The Lexus was fitted with Z-rated tyres for those situations when it's driven fully laden at its speed potential for extended periods. You're right, it's something that's not legally possible here, so choosing a tyre with a lower speed rating is good advice.