I got my first Sl in 1987 (70 450 SL). I traded an 82 signature series lincoln continental. My rates went down!!!
For 2 cars my annual rate dropped about $200. Shortly thereafter I moved from Mesa AZ to Scottsdale AZ, my rates jumped almost $300 per year. I checked all this out with my insurer. The drop was due to the low accident rate experienced by SL owners. The raise was due to moving from an area of low cost accidents (fewer high end cars) to an area of expensive accidents (more high end cars). There are an awfully lot af variables involved in rates. Yours do not sound bad for the NYC area.