That's fine on paper but what about atty design (chamber size) and airflow? You start heating a coil in a confined space, the heat starts building, right? The greater the airflow, less heat and vice versa.
Remember we're talking about heat produced... NOT temperature... A BTU is the "amount of heat" needed to raise a pound of water 1 degree (F) at 1 atmosphere of pressure... Note I didn't say how "long"... It's kinda like wattage versus current... While they are linked, they are dependent upon other factors...
Could you calculate based on voltage and resistance the exact temperature? Sure... But the calculation would be EXTREMELY complex... As you say, it would depend on chamber size, coil size, air flow, etc... But again, BTU is just how MUCH heat, not the actual temp...
To go back to another analogy, think of it this way... I can take a 1500 watt heater (a little over 5000 BTU/h) and put it under my desk at work... Turn it on and damn near kill myself... LOL... On the other hand,
throw that 1500 watt heater in the middle of a 100ft x 100ft uninsulated pole building and I'd never even know it was on... It's still generating the SAME amount of heat... but it's impact on temperature is going to be quite different!