Yessir, Alk, IMO there are two possibilities here.
Heat and light on the underside is going to make the plant want close its stoma to prevent water loss from the additional heat and light. Closed stoma will inhibit gas exchange and thus less CO2 absorption, less growth.
The plant will 'sweat' out more water without utilizing the light to create any useful energy.
If the stoma don't close they'll remain open trying to compensate for the heat/light and over perspire leading to excessive water loss, which is just more water it has to work to put back in.
So either possible outcome, IMO, is a loss.