I don’t think lowering or raising the temperature changes the wattage, I think it changes how long it’s on for, and you run the risk of turning the current on or off.
Tosafos was referring to mitoch. If making cheese, which is ochel nefesh, is boneh, all boneh should be muttar mitoch. Tosafos felt compelled to say that ein hachi nami, and boneh on Y”T is d’rabbonon.