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Do you have a pyrometer? I just got one, and will be using it going forward after my less than stellar experience running tall 275/35 and 245/40 tire on too narrow a rim.
I tried all kinds of pressures and couldn't avoid the flex issues. I couldn't find a pressure that gave me reasonable temps across the tire. The rim issue exacerbated the problem, but the taller tire and increased sidewall height (and softer sidewall of the PSS vs. the Star Spec) is probably an issue on it's own.
Tires sound like they're well sized to the rim in your case. But there's a half inch more sidewall height as compared to your 18s in the same /40 size. Diameter is more or less the same of course.
I'd play with more pressure, up to 41 hot, as Rated M suggested, but I'd check the pyrometer readings as well (and of course feel). There's only so much pressure can do though. A half inch more in sidewall is going to have some impact. If pressure/pyrometer-based adjustments can't solve this, then the options are probably a stiffer side walled tire (NT05 or Star Spec) or a different profile. I know it's an expense, but can you run 265/35s without rubbing?
Guys I run with at the track who run the RS-3 (I was considering this tire) love the tire. But they did also say had a softer sidewall compared to Star Specs or NT05s, and some had gone with a lower profile to dial things back in (always compromises).
I've decided to run 235/40s and 265/35s with NT05s on my 9" and 10" rims for the track vs. 275/35 and 245/40s as the increase in section width is more or less negated, IMO, by the increase in flex and associated issues (and lack of confidence and traction, and 5 seconds of lap time) the come with it. I moved the PSSs to street duty only (and appropriately sized rims).
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