Well, it ain't rear toe either... I've had "more rear toe = more stable" preached to me but for me it has never done anything but make great-handling cars either criminally unresponsive at turn-in, or even make them behave more nonlinearly and diabolically (AP1) even in a straight line. Dialing in rear toe-in is not going to fix what you are describing, which to me still sounds like a combination of the 55/45 F/R weight distribution we are stuck with and tires that aren't particularly grippy.

I'd stick to zero to 0.1 degrees total rear toe-in. Adding rear toe-in isn't going to magic you more grip, but it will make the car less responsive and less fun to drive.

It's not a matter of more grip, it's that when it loose grip it's sudden and not progressive. For sure it's not the PS4 that have not enough grip...
If you want responsiveness, start changing all that shitty bushings our car have.
My Fiat Panda has stiffer bushings.

As for toe, just re-read your original post, IMO 0.1 degrees total rear toe would be fine but I don't think you'll notice any kind of profound change really.
That's been my experience with this car, I've run both 0 and 0.1deg.
On other cars with heim joints and aluminum/delrin bushings, I've found too much rear toe (in excess of 0.3 degrees) to suck, bad, for everything, but 0 to 0.1 even 0.15 degrees no probs...

Losing grip suddenly and not progressively can be a characteristic of how the tire grips. Some tires have a lot of latent grip after wheelspin or sliding begins, and some have less. MPSS I had on the FD RX-7 were kind of the worst in this regard after 2 years on them, any amount of wheelspin and they instantly lost all lateral grip. Not progressive at all...

Dunno what you can get for more trackworthy tires, but you might consider something like RE71R, A052, RT660, ZIIIor if Michelin is preferred, Pilot Sport Cup 2?

RE bushings, I havent done much with my BRZ there, just polyurethane inserts for the rear subframe, which helped a bit with left/right transitions. Pretty happy with it as it is for street/track at the moment but may get poly front control arm bushings before next track season...

For what it is worth, I took "stepping on the gas" as basically going full throttle with the wheels turned. That is always going to cause the car to step out. If the car is too loose, feeding throttle while unwinding the wheel, then that is a different story.

Personally, I run about .16 rear toe in. My setup is 245/40/17 A052's on 17x9's, about -3.7 up front zero toe, - 2.2 rear, 7k/7k springs. And the car tends to range from really good to slightly too loose on corner exit. I've never ran 0 toe in this car, but I think it would be diabolical with 0 rear toe.

Thanks everybody, I found some experience even on the UK forum, and it seems that rear toe in is the culprit. I now have to decide if 0.1 or 0.15 deg per wheel.

I should add that my .16 is toe rear toe, so about .08 per side.

I wouldn't go over .1 per side.

Which translates to 1mm toe in per wheel

The Michelin PSS i ran always have this issue. Problem goes way when i run my 200TW tires. I accepted that it's just a tire grip issue and moved on.