I concur with 10:52. High Performance tire is bad in snow even on FWD or AWD car, let alone a RWD Lexus GS. Get you a set of snow tires next winter and you should see an improvement. Also, due to the fact that the GS is RWD, you should put a couple sand bags in the trunk as close to the drive axle as you can to get more grip in snow.
If you put the PZero in a BMW 5-series, you will get the very same result in snow. The fault lies not with the car, but the tires.
Additionally, you're not supposed to be surveying the TIRES, but the CAR. and at the top it says "what has gone wrong with the car: Nothing" yet you put a big fat frowny face on this nice car.
Additional, The car is not well balanced this is why any tyre on the car would have the near-enough the same outcome. As If you put high heals on Carl Lewis and you had the best running shoes in the world, he would still beat you to the 100m finish line! Same principle.
Just try running in high heels sometime and you won't make that kind of argument!!
I owned a Mk2 GS300 Sport for 2.5 years. I put Eagle F1s all round, and the car was fine in adverse weather. OK, snow was treacherous but the VSC works fine. In the rain, it felt very sure footed, the GS chassis is well balanced. You must have something wrong with yours if the VSC is kicking in that much. I've hardly ever had the VSC kick in, and I drive pretty fast.
Oh, I went on to put coilovers, chassis braces and stiffer anti roll bars on the car. Let's just say it sticks to bends now!