>>I read that the plane changes shape a little as it heats up from the friction of flying so fast. As a result the pilots never determined how fast it could actually go.
It doesn’t change shape. But the skin panels do expand due to heating so it is built to permit that expansion between joints so that it doesn’t warp and change shape.
That, and the cone inlets to the engine can move quite a bit depending on speed and altitude to adjust for proper fuel burning.
I’ve also read that the planes constantly leak fuel because no sealant will withstand the heat. On one picture taken from a tanker you can see the fuel leaking from the seams.
I’ve read that there was some minor reshaping on the leading edges/Chines due to the extreme heating of those surfaces. Every aircraft was unique, and slightly different after each flight.
Apparently very subtle differences that only an experienced sled driver would notice.
I can’t speak to the accuracy of such, just that I’ve read it.