There is duty on a car if it is NOT made in the US (which the NSX is not, so yes there is duty when you bring them into Canada)
The 15 year thing is for RIV.
If its 15 years or older you don't have to process the car through RIV (driving lights, child seat anchor, and French SRS sticker)
Hidden gotcha. There is a $100 A/C tax paid at the border as well.