I ordered tires this evening. After much research I decided to go with the Bridgestone Potenza Sport. I had been planning to take the well-trod route of Michelin PS4S's, however they're not available in New Zealand (and importing tires was just going to be too expensive to make sense) in any of the sizes remotely close to what I need (I confirmed this with Michelin NZ directly), this was a surprise especially given I'd checked on their NZ website that those sizes were available here 👎
This meant I then needed to figure out alternative options. In Michelins that meant PSS's or PS5's (no Cup options available in NZ in the sizes I need). The PSS's are too old, and the PS5's while by all accounts very good had me a bit concerned about the softer tire wall than the PS4S. The RE050A's that I'm running currently, are fairly stiff in the sidewall department and while objectively terrible compared to modern options I am used to the stiffer sidewall. By moving to 18's I'm already introducing more compliance for a better ride and road stickability (that's the technical term 🙃), and I was a little concerned about also moving to an even softer sidewall as well.
I then looked wider, and after a bunch of research, looking at what is available in NZ, settled on the Potenza Sport. The Potenza Race would be an even better option I think, but they're not available in 18in options. The RE71-RS's while the stickiness would have been giggle-worthy, don't give the same degree of feedback and would be impractical in wet conditions, so the Sport it is. TireReviews.com have a good review of it and on dry handling it's very close to the PS4S, slightly better in the wet, and ranks worse on comfort (due to the firmer sidewall), slightly worse on noise and worse on wear. The wear doesn't bother me as I won't do enough km's anyway, the firmer sidewall I'm okay with and the noise will still be a lot better than the RE050A's anyway.
This is all splitting hairs of course, but might as well get the best option I can.
Big shout out to @Palazzo for suggesting Costco. Bridgestone NZ direct have the best price out of the rest due to a special deal currently which comes to only $2376.32 NZD for all 4 tires fitted. Costco, for exactly the same thing, is $1379.94, plus in addition you get a $360 store credit as well. Given we regularly shop at Costco for essentials this means the end cost for the tires comes to $1020. Which is quite astonishing to me. That's significantly less than I paid for new performance tires for my 318i in 2007!