Although I'm in NZ now I'm from the UK, and I insured - let's go with "a number" - of high performance cars before I left. What I found as I worked my way through the ranks from what I could afford as a 17 year old, through French hot hatches (and while they were still good enough, I went back for more) to German Uber-wagons and low-end exotica, it seemed to me that the hatches carried the highest premiums per BHP / £ of value.
At 26 I went from a Renault 5 GT Turbo to a 911 Carrera 3.2 Sport at over three times the value, and while the premium increased a little as I recall it was only a little. Moving forward a few years I had a 306 GTI-6 and a Carrera 4 at the same time and paid pretty much the same for both - limited miles on the 911, but not to the point I couldn't use it. An original RS6 Avant with a remap and a supercharged Mini Cooper S were also pretty similar when I had both of those at the same time. I also had a 944 Turbo tuned to 400BHP and set up for track days with two sides of an A4 sheet of mods that totalled around three times the car's value, which was very reasonably priced - comparable to when it was standard in fact.
I believe cars that made it into group 20 (is that still how it's done?) and get a bespoke quote were often cheaper to insure than cars that fell in the group 16-18 range - presumably those were crashed more, and possibly that was due to affordability.
I did think of the RS Cosworth when I read the first post in this thread, but the issue with those was that they were so attractive to thieves, and security was pretty poor in the late eighties.
Anyway, I have no idea how the S Plaid will be viewed by insurers in the UK, but I fear it will be high in NZ. Insurance here is generally laughably cheap compared with the UK, but that's largely because it is not mandatory, so if the quote is too high the customer has the option of just not bothering at all. Our M3 SR+ is the most expensive car to insure I've ever had in NZ, coming in fully 40% more than a 550BHP supercharged V8 Aussie "Ute", which was insured for quite a lot more than the value of the M3. That's equivalent to about £1,000 for comparison with what folks pay in the UK.