You're just going to have to pardon me if I'm using the terms "undershoot" and "overshoot" to mean a "situation which actually causes a comfort problem" as opposed to normal system operation. There is practically no residential system at ANY mass level that does not have a fluctuation of room temperature of at least +/- 1 degree typical.. plus or minus 2 degrees is still normal and acceptable operation for very nearly all systems and certainly the vast majority of radiant systems of any mass level in the situations we are discussing which represent aggressive load shifting scenarios. Heck, many simple thermostats have a three degree swing standard, again without regard for mass. Every forced air system in america will swing by 2-3 degrees minimum. You think it's because they using extra high mass fat air?

There is no value in defining "under/overshoot" so absolutely that it's a useless term, other than making it so you're "not wrong". Other than in a small handful of exceptional circumstances we have already discussed, there is near-zero practical value in putting low mass radiant over a slab you could instead use for radiant directly.

re the slab sensor: If I don't let a slab get stone cold during an off period (if I keep it at room temp) then the room temperature will not undershoot SIGNIFICANTLY due to the natural reduction in heat loads, pretty much ever. Not because room temp is an adequate temperature for a slab to maintain room temperature.... obviously, but hey, thanks for the condescension.... but because I will be able to recover the slab temp from that baseline fast enough to prevent any discomfort issues. if some rare construction situation was present for which that was not true, then simply raising the minimum temp of the slab a degree or two would solve almost any issue without any objectionable or easily noticeable effect on room temp. In short undershoots are a problem that is incredibly easy to address in almost all cases.