Think of this another way. The rolling diameter of a 18in wheel plus tyre will be similar to a 17in wheel plus tyre, perhaps 2-3% larger
You could use "17in" space saver wheel provided you stick to the recommended speed and distance
To complicate the issue, the space saver will only on the rear of an Evo as it wont fit over front Brembo brakes!
Solution. a clueless breakdown service