After some research concerning those aircraft that were decidedly subsonic
in level flight (no pushover from altitude to gain greater speed), it would
appear mach effect is the overriding concern. The last low altitude record
before the transition to high (F-100, with several ... F-86, F-4D ...
previous to that) were all done at the Salton Sea. Hi temp (higher TAS for
mach) and low altitude (-227 MSL), delayed transonic drag rise.

The PsubS bulge doesn't occur until you get into the cleraly supersonic
designs. Then it behooves a "low altitude" record to occur as high above
MSL as possible. Hence the sageburner and later Greenamyer efforts in the
high desert (less IAS, more TAS, 988 mph for Darryl ... great film by the
way).

Bottom line, in our running discussion, I now find your argument compelling.
I was incorrect.

R / John