My only experience in this matter is actually going the opposite direction: a used 414ce that I bought a few years back had been strung with medium-gauge strings, whereas Taylor recommends light-gauge strings for the Grand Auditorium (GA); I switched back and made the appropriate adjustments on the truss rod, partly because that's the recommendation of the manufacturer and partly because I find the light-gauge strings more comfortable to play. Frankly, I didn't note any significant difference in tone or volume -- but I was also going the opposite direction on a smaller guitar body...
To take a poke at answering the root of your question from a largely scientific/physics/engineering perspective: I believe any differences that could be perceived would be perceived as tonal due to relative loudness: lighter-gauge strings being 'thinner-sounding' and somewhat quieter -- largely for the reasons given by "SF" (the ability to impart energy to the top). Larger (i.e., thicker) strings have more mass, which in turn will transfer more vibration momentum better.
All that beings said, though, I would surmise that, like the GA, the sonic difference due to string gauge might be bordering on (if not entirely) negligible for the Grand Symphony (GS) body as well, which has only a slightly larger (1/4-inch) lower bout than the GA; all other dimensions of the GS are the same as the GA. That means the top has only a bit more surface area. The medium-gauge strings will definitely be up to the task of driving the top adequately, but I would guess that the light-gauge strings might only run out of gas at the extremes of the frequency response spectrum (i.e., the lowest lows and the highest highs). So confirmation bias aside, one is not likely to notice any significant difference in tone so much as a small difference in volume -- or perhaps in practical terms, muted tone that requires more effort in strumming/plucking the lighter-gauge strings to get the same volume as medium-gauge strings. In the end, it's probably a bit of a "chicken-or-the-egg" discussion...
I would opine that this topic maybe only has relevance when applied to the truly large guitar bodies (Jumbos, Grand Orchestras, etc.); those guitar bodies in the midsize range (GA, GS, Dreadnoughts) have probably been determined to be optimal with their respective recommended string gauges, but will do fine with any similarly-gauged string set.
And in the end, Taylor's string recommendations are just that: a guideline that they provide so those of us who aren't luthiers have a starting point. Like Edward points out: in the final analysis it comes down to what your ears hear, and what your fingers find comfortable to play.
