I agree with Bluter. With that being said, however, you might still get a benefit - if your lighting setup is overheating the root zones, Reflectix would provide some slight insulation between the one and the other.
But I'd attribute any increase in productivity to just that (above). If your light is intense enough to make it through the canopy, to "ground level," get reflected by a poor (in regards to light reflectivity) reflective surface... then on through the leaves to the chloroplasts just under their top surfaces, lol - then the plants are already maxed out as to the amount of light-energy they're capable of processing at any one time.
But I'd attribute any increase in productivity to just that (above). If your light is intense enough to make it through the canopy, to "ground level," get reflected by a poor (in regards to light reflectivity) reflective surface... then on through the leaves to the chloroplasts just under their top surfaces, lol - then the plants are already maxed out as to the amount of light-energy they're capable of processing at any one time.