I haven't tested this certain brand but similar insulator pads consisting of this silicon rubber reinforced with glass cloth. The major difference to mica is, that they are thicker and softer! The softness can be a problem, if you want to mount a TO220 case on such an insulator pad using a screw. As the mounting force is rather inhomogeneous then, namely concentrated arround the screw hole, the insulator formerly looking like a symmetrical rectangle tends to change its shape...
This strange behaviour is supported by the fact, that tightly turning the screw will finally act a torque on TO220 case, making the insulator pad furtherly 'smear'. At this very moment you begin to fear, that the pad is tearing under the TO220 somehow and that case and heatsink will eroneously come into contact somewhere near the screw hole, and you loosen the screw again...
As consequence you cannot apply that amount of torque (means pressure) you actually wanted!

If you have enough 'headroom' regarding heat dissipation, then it's a better way to turn the screw not so tightly and take into calculation a somewhat increased thermal resistance, means a somewhat worsened heat transfer performance. Then, everything remains in shape...

A nice remedy would be the use of a clip, instead of a screw. This would even help to apply a much more homogeneous pressure and by this improve heat transfer.

Kai