The only reason why it COULD be cheaper, is if you drive it less, hence reducing the risk of incident. Which is why they probably document that you have a second/daily driver.
The fact that it's rare and or has a higher than listed book value, could mean a higher limit (which would only effect your comp/collission rates really). A higher comp/coll rate would also raise your premium. Insuring a car is like insuring anything else though... If you put a 30k limit on your spec b and total it - that doesnt mean they give you a check for $30k.
Pending the driver, the majority of the policies premium goes towards liability coverages regardless.
It's possible that your buddy with the type R was overpaying a bit with his older carrier, so the Haggerty premium is not as great as it would seem.