The way I look at it is to consider the difference between the W fare and the R fare. That's the cost of your insurance policy, should the GPU not clear. It's the same calculation (with different inputs) when you buy a W fare over the cheapest, simply to get the hope of your GPU clearing.

Yes you do get priority for clearance but not much - you still need PZ space.

And, no, you can't use the Polaris lounge if in PE but, assuming you are *G, you can use the *G lounge.