1. Buy a Ford FG XR6T and take it for a trip to Horsepower Factory for a Venom Stage 5 kit. Should see 525kw+ @ the engine, and 420+kw at the wheels.
2. Buy a Holden LSA and take it to HPF for the Harrop Stage 4 kit. Should see around 570kw+ / 1000nm+ at the fly, and over 400kw at the wheels.
3. Buy an A45 AMG and get the Stage 1.5 Willall Racing kit. Should see it jump from 265kw/450nm to ~335kw/610nm.
Again though it depends by what speed metric you want it to be fastest. Fastest off the line, over the 1/4 mile, or in top end.
Off the line, you can have one of the quickest street legal cars out there, but if a Tesla P50D happens to roll alongside you and has Ludicrous mode enabled, you are getting eaten alive.