Madanark, I don't know of anyone in AU running 18 x 8.5 wheels, and there are a number of reasons for this.
Yes you'll be able to get them engineered when you get the S/C done but I'd go and have a chat with the approved engineer that you’re going to use for the S/C to ensure that they'll pass them before you head out and buy the rims. Check your clearances against the standard tyres remembering your stock rim has a 55mm offset. The standard 205 x 45 x 17 tyre is 616mm in diameter. A 225 x 40 x 18 tyre is 637mm in diameter and if you chose a 55mm offset would be 10mm closer in both directions on the inside. Check your lock clearance and remember vertical travel.
Performance wise the car will be slower than using the stock wheel. Besides the increased weight in the tyre & rim that needs to be turned there's a 5.5% increase in rolling diameter. If you have been reading Miata.net forums remember that the American's won’t notice the drop in performance like we would as they have a 4.1 diff ratio to our 3.7.
Your choice of 17 x 9 with 245 x 40 tyres for track days is a bad call before supercharging.
First up they don’t qualify in the modified 2B class where every other modified NC’s runs
as they are over the width limit by 1”, so you’ll need to run in the SV class which has no limits to the modifications you do as long as its road registered. This class is for those with plenty of $$$ and will be the class you go in when you fit the supercharger.
Secondly you need to be very careful in picking the correct offset to clear the inside body work. My 16 x 8” rims with a 50 offset running 225 x 45 R-Specs (608mm dia.) just touch on the inside of the body work over the Wakefield bumps. It’s also lowered around 40mm which decreases clearance. A 17” x 8” rim with 245 x 40 tyres is 628mm dia., so would be 10mm closer at the top and 5 mm closer to the inside than my tyres and would hit the inside bodywork for sure. You’ll probably need a minimum 35 offset to the rim. This then produced the next problem, the guards will need to be rolled to get clearance on the outside (mine are slightly on the front to clear) and most probably cut to fit flares to cover the tyres. If you measure 40mm out from your standard tyres you will see where the 245 tyre and 35 offset will come to on the car.
Another thing I’d like to point out is that the quickest NC around Wakefield uses the stock NC rims and runs 205 x 45 R-Specs which backs up the theory that as soon as you throw big wheels at the NC, or any MX-5 for that matter, the weight kills the straight line performance.
Time for some more head exploding time.
