In my opinion, rather than going down a size, I would go for PZ4 with OEM sizes, but that is obviously a personal preference. That said, it is a shame that Pirelli has not (yet?) introduced specific sizes for the GTR on the new PZ5, which, judging by the comparisons, seems to be equal if not slightly superior to PS4S on both dry and wet surfaces.

For many decades now I’ve never been happy with any Pirelli High Performance tyre, as I’ve found their quality control is compromised from a balance standpoint. Meaning I’ve never purchased a Pirelli ZP tyre that didn’t need a hell of a lot more balancing on a Hunter Road Force machine than a competitor from Michelin; which has been my go-to brand for many years now.

There’s one caveat: and that is the Pirelli Sottozero, which is a brilliant Winter ‘High Performance’ tyre; just don’t expect it to handle as well as a Summer ZP tyre obviously.

Perhaps I’ve just been unlucky.

I agree — and you’re definitely not the only one to run into this .

One could joke that Pirelli’s long experience building “intentionally imperfect” F1 tires may "accidentally" spill over into the road lineup

That said, the Pirelli Sottozero (Series II and III) are genuinely strong winter tires specifically on packed snow and in temperatures above ~20°F.
In my experience, however, the Michelin Pilot Alpin PA4 performs noticeably better at very low temperatures, well below that threshold.
So the right choice really depends on where and how you drive during winter.

Another practical limitation with the Sottozero is size availability. Pirelli no longer offers many sizes that properly fit our cars.
Michelin, on the other hand, still supports ideal fitments:

265/35R19 front for GTC
305/30R20 rear for GTC (near-OEM)

315/35R20 rear for GTRs with 265 fronts (the GTR front rear rev per mile will still be below the "agreed" 5% delta

And 335/25R20 (very low availability) which I personally like for dry but extremely cold conditions (20°F and below) when serious winter performance driving is still on the table

IMHO, looking at the performance, temperature range, and fitment standpoint, the PA4 remains the most versatile and confidence-inspiring winter solution for these cars.

Have to agree about Pirelli tires. I'm guessing that, since the French were one of the first to colonize African nations that have rubber trees, they have the most experience and thus better rubber technology. I've found that Michelin seems to have the longer-wearing tire in a given class.

I ran the PA4 winters on my C63 S and agree that they were very good ... but 500+ HP to the rear wheels still means "I want to kill you all the time" in snow (and in non-snow if you don't respect the throttle). #;-))

On our C43, I picked the Michelin X-Ice tires over the Pirelli Sottozero due to reported better wear with the X-Ice and the fact that (as reported) the Sottozeros only use the special "ice adhering" rubber for the top couple of 32nds on the tread and, once that wears, the ice-type traction goes way down. The Sottozeros were are also reported to wear excessively when running on non-snow/ice surfaces so, since we mostly have winter rain, the X-Ice made more sense. Mine are stored in a garage over the summer and I'm now on my 9th winter ... though we only drive the C43 about 8K km (~5K mi) per year. So far, they're not showing any signs of noticeable rubber deterioration and still have about 80% tread remaining. I'm sure they'll deteriorate before the tread wears out.

I’m adding a link to a video I recorded last winter demonstrating the Michelin Pilot Alpin PA4 in real-world conditions.
The video includes multiple segments showing excellent grip on dry tarmac (no snow on the surface) at ambient temperatures of 9°F, 13°F, and 21°F.
Several clips include full-throttle pulls with a 500+ HP setup in 4th gear, as well as sustained high-speed driving.

The tire setup is 265/35R19 PA4 up front and 335/25R20 PA4 at the rear.
Cabin footage displays the outside temperature in the upper-left corner of the speedometer cluster (slightly blurry at times due to camera movement).

Video link:

Note: I’ll keep the video available for a few days, as I’m running low on space on this Vimeo account.

That is a great video. Thank you very much for posting it.