having worked in insurance in a past life, most insurers either start with a min policy price and add up the rating factors onto this price, or start at a max price and deduct rating factors. Take your 350z out the equation, the issue with your price is that you can't allocate your no claims bonus to the MR2.
Mirror policies only tend to be slightly discounted and will be nothing in comparison to your no claims.
Most insurers wont recognise anything to do with your other car other than a “2nd car discountâ€
If you are sharing the miles between both cars did you reduce this on your original quote? ( bare in mind that miles per year is a rating factor, too little miles can put your price up as can a lot of miles )
I once done a quote where we spelt the customers name wrong just before we sold it, when we rectified this the price went up! Never ever seen that again but it happened!