My 17 GTS doesn't have PTV+ and I'm extremely satisfied with the handling. I haven't driven one with it so I can't say if it's worth it or not. I would like to know how much it improves the handling in the snow since my wife will be driving it in the winter for the first time and she's right foot happy. Is it true that it cannot be retrofitted?
In an open differential, torque flows to the faster spinning wheel. So if you’re in a pile of snow and one rear wheel digs itself into a little hole and just spins, the other rear wheel won’t get any power to help pull you out. This isn’t quite as much of a concern in an AWD car as power will still get routed to the front. And there’s also traction control systems in place (the interaction between all these elements is something of a black box to me). PTV+ should generally provide an advantage in snow, but it seems like the car still has enough methods of keeping traction.
Which leads to tires - if you’ll be seeing lots of snow (and/or very cold temps) you should have a dedicated set of winter tires. Moving torque around still only helps if the tires can grip in the first place.