(It's rather late; I'm tired, and this post rambles. Sorry.)

Unless you consider the particular mach cone angle which has the shock traveling parallel (or just a little off, in the wrong direction) to the leading edge - hopefully only briefly! Then the pressure can be a lot higher than ambient thereby increasing the heating a lot.

Using cosine weep angle approximation looking at normal flow to the leading edge helped in designing reduced drag wings passing through transonic and M1 by quite a bit, This allowed the creation of the first supersonic aircraft. But this approximation only works well for wings of some aspect ratio, not so well at the tips and not at the root. Our fins are pretty stubby. The formula is a simplification, but not quite accurate.

The fins of the early supersonic planes had sufficient sweep that the leading edge stayed behind its own shockwave (so the wing stayed in subsonic flow). But the higher the airspeed, the more swept the shockwave angle. At sufficient airspeed you need either a considerable leading edge sweep angle or need to design such that the leading edge is in advance of the shockwave from nearer the root. Then you might get something that looks like a Nike fin, or possibly the wing of the F104 Starfighter, or the fins of many sounding rockets.

For almost any reasonable fin on a high supersonic rocket, there is going to be some airspeed for which any chosen sweep angle is well off optimal. One wants to choose a sweep that minimizes time spent under such conditions, weighted by atmospheric pressure. IMHO, of course! That's for altitude, not just for speed.

For max speed, just burn as much propellant as fast as you can without CATO. So there is less [integral drag over time] to subtract from top speed. Also use a nozzle with exit diameter near total airframe diameter. That minimizes drag while the motor is burning.

I was looking for some pictures from old supersonic and hypersonic windtunnel tests where the fins (or wings) melted. The melting initiated at the tip of the nosecone, and at the leading edge of the fins right near the root. For some reason I couldn't find the pics tonight. Sorry!

Gerald