Have you considered Moda Bella for the sashing? It's a 60x60 broadcloth, like the regular Kona, but it has a much softer hand due to more tightly twisted threads and a nice silky glow (not a sheen) to the surface.
Kona's roughness is due to loosely twisted threads and it does soften up quite a bit after washing and machine drying. I've used both Kona and Bella in the same quilt, and they play very well together, despite their 'twisted' differences. :lol:
Once your quilt is finished, I don't think the thickness differences would be very noticeable, if at all, but if you don't like using resular Kona, I think maybe you should take a look at the Bella over the Pimatex. I can send you a sample of Bella and regular Kona in the same color if that would help you decide.
Just a note, because of the prominant grain of the Pimatex (unequal thread numbers in warp and weft), cutting can be tricky and stretching can be significant. Something to consider for a show quilt.