Some people take their shit off the road.
Anywho, i know that when i first got my skyline i wasnt used to the power it made, and i had been driving for 18months before hand, i also know that when i first got a car (lancer) i got into some hairy situations, these would of been greatly worsened if i had been driving my skyline at the time.
Restriction on engine size would be sufficient and easier to police, kw/tonne ratings are too difficult to enforce as the police would need to keep a database of every make and model to refer to. something like <2000cc naturally aspirated, unmodified.
Sure some cars will go faster than others in this category but the same can be said for the restrictions on motorbikes, but it serves its purpose which is keeping the majority in proportionately slower cars.
Its not gonna be a complete solution but its a start.
Plus those vn kids wont be able to drive them;)