Today I learned, thanks to this thread, that the RPF1 legacy is expanding, still, in 2023!
Soooo... yeah... about those 18x11's... Do want.
You know, I find it funny, us Supra guys almost always seem to go with tires that are wider than the wheels, yet the Miata world seems to square them up a lot better. My race setup is a 205/50 on a 15x8. My winter setup is a (I think) 195/60 on a 15x8. I don't think a 215 is a bad match for an 8.5" wheel, but personally I wouldn't go any skinnier than the stock tires were. I had a pretty bad experience with a 255/40 on Mk4 17x8 wheels, you could actually feel lateral movement in the tire, mid curve. As in, you'd turn in, it would feel good, then the wheel would move from the center of the tire beads to the outer edges of the tire beads, so you felt this really strange transitional period in every turn. Felt very balloon-y.
A 255/40 is way too wide for a 17x8, or even a 17x8.5. Stock was a 235/40 on the MKIV 17x8 and most folks do a 245/45/17 front and a 275/40/17 rear on MKIV wheels since that matches a ~25.8in (IIRC) rolling diameter front & rear, and a lot of decent tires are available in those sizes even to this day.
Most tire makers are including a width range with a given exact width the tire was designed for, which makes this process a lot simpler. Go too narrow on the wheel and yeah you're dealing with all that weird wiggling and tire instability, and the less sidewall height there is (like with a 35 or 40 series) the more spectacularly terrible this gets.
Go too wide on the wheel (stretching the tire) and you're stiffening the sidewall a ton which could be good or bad depending on your goals. For steering feel its great, forward traction on a drive axle or ride quality on any axle, it's terrible
Yeah, we Americans have a cultural belief that wider tires are always "better" performing somehow. More often, the opposite is true. We used to mess with this stuff all the time at the Proving Grounds. With no increase in power, wider tires almost always slowed lap times. Even for offroading there is this cult of wider tires. The reality is a higher ground PSI of narrower tires provides better grip and greater directional stability on a broad spectrum of low friction surfaces. I shake my head at all the "tough" trucks driving around in cities with 40 series huge rubber that sticks out 3" beyond the fenders. Those would last about 3 minutes offroading. Stockish size and high quality rubber will get you REAL performance, or you can go with the mere LOOK of performance.....
It all depends entirely on why they're going wider.
If they're going wider because the car needs more lateral grip and is pushing like crazy in turns or struggling to stay on a racing line, wider tires of the same compound will make a huge improvement. But with more lateral grip you need more roll resistance in the suspension somewhere, more strength in all the steering parts, adjustments to the suspension to deal with the higher unsprung weight of the wider and heavier wheels and tires, the list goes on.
If the car in your example is holding a racing line very well, not pushing in turns, and generally staying together throughout a lap then the higher rolling resistance and driveline losses of the heavier/wider wheel & tire package will slow the car down, especially if it's a really fast track with some big straights.
Similarly, RWD high horsepower I totally agree there's a trend for folks to ignore the factor tire height plays in forward traction and just go wider, and going to a wider tire of an inferior compound will almost always be about the same or worse than a narrower tire on a good compound. Best is a wider and taller tire with a better compound. Is there a limit to how much additional width helps? Yep. Are most sports cars, especially 90's and 00's era sports cars set up with wheels and tires far too narrow for their performance abilities? Also yes.
We're in 100% agreement on the over-tire big lifted trucks. Back in my industry days, my shop buddies and I would finish off SEMA by getting a few too many drinks in and wandering through the displays of hackjob, over-lifted brodozers with the 24in bling bling wheels with knobby off-road style rubber band sidewalls, pointing out the terrible pinion angles, total absence of suspension travel, and sometimes total absence of other components like brakes, brake lines, or driveshafts. I remember one year there was a lifted blingy brodozer like that missing it's front driveshaft, but the builder had a sense of humor and painted a bluetooth symbol on the front diff's driveshaft flange. We laughed our asses off at that one.