How relevant is Mach 2+ performance these days - how relevant was it at
all
?

It was never tactically relevant.


I can imagine high speed being useful when intercepting the odd Foxbat or
two, but otherwise - how often would you require such high speeds ?


An F-4 could theoretically reach launch parameters for a mach 3, 70,000'+
target doing about 1.4 at 36,000. Biggest problem was controlling target
aspect in the horizontal. More speed would have helped some there.

Speed can be useful in minimizing raid penetration and increasing AA missile
LARs. It's also very useful when leaving hostile territory, albeit that's
typically at mid altitudes where Q vice mach is the controlling factor.

R / John