Yes, your contractor is correct.
User manipulation of the engineered operation of this equipment as you described is not recommended. This is commonly referred to as equipment abuse and creates the potential for short cycling of the equipment, uneven steam room temperatures, premature failure of the unit, and warranty nullification.Are you trying to increase the temperature or create more fog in your steam shower?
Steam is vapor and vapor is clear so increasing the temperature will not necessarily create more fog, if that is your goal. Super heating the room will likely make the fog disappear since fog is created when hot meets cold, like the warm ground meeting the cool morning air.
Consider adding glass, windows, natural stone or marble, or similar wall materials which increase the heat loss factor of your steam shower environment. This will aid in the creation of thick and foggy steam. Another wonderful way to increase the fog factor is to turn on the water - the cooler the water, the thicker the fog potential. Running water is also an amazing addition to the ambiance of your steam shower environment.
Proper sizing and design of your steam shower environment will afford you the opportunity to create an environment which satisfies your preferences and allows the equipment to operate in an optimal fashion. Check out this design tool to learn more...http://www.steamsaunabath.com//catalog/all_about_steam_rooms.php#overview