Well, a change of plans now. I was told by someone (whose opinion I generally trust) that the PVC shower liner was the way to go. But, given the reactions here, I talked to my tile guy and he agreed...no liner, just durock, floated seams and RedGard. That means I'm stuck with $100 worth of Shower liner (apparently what they use to make onsite shower pans). But I'd rather do it right, and that's what I get for jumping too fast.
The liner was to go behind
the durock. Tile was planned all along. As to the receptor, it is a good one, but I bought it from a local supplier for just under $700. I wanted something to last my lifetime
Thanks for the heads up. If you hadn't expressed concern, I might have moved forward with the liner. And I will contact Kohler to verify the shimming.
This forum has helped me in many ways. Thanks to both of you for your inputs.