I think the idea with using the resistor on the IAT sensor is that you force the IAT value down to it's minimum sensor value and then create a column in the tune for the "Timing Total Cor. (Charge Air Temp)" table for the extremely low value the resistor creates with a massive timing reduction. So when button is pressed, IAT reading goes down to -40C and you pull a load of timing.
You're right not to want to pay for the tune, it's unnecessary.
Still would be much nicer to be able to use the MDrive steering wheel button rather than a switch hanging out of your dash. I'll have a look at see how involved that would be.