Heat doesn't speed up any _chemical_ reactions when making kava, nor does it make kavalactones more soluble (cause they're really not very soluble at all).
Heat does, however, help to dislodge starch, oil and kavalactones from the plant fibers. These 3 things form a pickering emulsion, rather than a solution. The rate at which these substances are dislodged from the solid plant fibers DOES scale approximately as an Arrhenius equation (that is, higher temperatures result in stuff coming out of the plant fibers faster.)
I'm not convinced that boiling water really damages KL's all that much. However, really hot water CAN alter starches, and shift the rates at which oils, starches and KL's are liberated from plant fibers. When I make kava with super duper hot water, more KL resin seems to get stuck to the strainer, the inside of the blender, the bowl, and so on. I think this happens not because more KLs are knocked loose, but because the starch granules burst at high temperatures and release their amylose into solution, leaving fewer intact starch granules to emulsify the KL's.
Melting the KL's does not seem to matter, so long as the KL is well macerated into small particles in the presence of starch and water to form a suspension. This is why smashing up the root is so important. You gotta get the oil, starch, water and KL's to come together. The abrasive woody fibers and the kneading action (or blending) makes the magic happen. Just plain soaking doesn't cut it.
Ever notice that kava made from super hot water is slimy? That's the busted starches, all gelatinized like.
tl;dr- science. Hot water is different. Not awful. Not awesome. Try it if you want.