If that were the case then the mach-10 Kinzhal would be harder to hit than the mach-5 Kh-32.

But the interception rate for the Kh-32 is basically nonexistence (<1%).

The Kh-22/32 is why mach-5 + maneuverability is the current goal of offensive missile systems.

The plasma has complex interaction with radar, it's not stealth as in entirely invisible just chaotic scattering and reflections. The result is a jamming effect preventing a definite intercept solution.

On the other hand the plasma shows up on satilite based IR tracking systems.