Heat might actually break down the plastic a bit, so you can leave your discs outside in the sun to soften them. I had a friend who used to leave Wizards in the back window of his car prior to SSS plastic and it really did seem to work. His Wizards were crazy/soft compared to the roof-shingle stiffness they started with. It took a long time, though. You had to leave them in the car for quite some time.
The story I heard about Gateway creating SSS plastic was that George Smith was Gateway-sponsored and liked a really soft putter, so they were leaving Wizards out in a practice basket in Dave's yard to soften them up for him. They decided to just create a plastic that soft out of the box for him, and it sold.
I always assumed that was where the microwave myth came from, trying to quickly reproduce the sunlight trick. I think the sunlight either breaks down the plastic or it's the process of heating/cooling repeatedly that works. The microwave seems like a really bad idea.