Uh, the stem temperature will be the same temperature as the wheel since it is attached to the wheel, and it's a metal stem, so why should it be a different temperature?
It may lag a little behind, but basically it will be the same temperature as the wheel, ergo the stem and sensor will be the same temperature as the sensor inside the wheel. The inside sensor won't be exposed to the air moving around the wheel, but since "metal is a better conductor of heat than air" that shouldn't affect the temperature of the stem/sensor much.