will a modest tonearm (say a rega) + a modest mono cartridge (say in the $500-700 range) perform BETTER on mono recordings than my (top-of-the-line) tonearm + (top-of-the-line) stereo cartridge?
Yes for the cartridge. Check out a Shelter 501 mono and you are there. A Mono Cartridge is for Mono Recordings and a Stereo Cart. for Stereo. To compare a cheap Rega Arm with a Top of the Line Tonearm is a different Story.
Try it with a Shelter 501 Mono cartridge, it is not expensive and you will hear your Mono's in a real good Quality.