The letter, if I recall correctly, came from MINI Canada, not the dealer and was in the form of a "voluntary recall". I do not have the letter in front of me but I think there were some dire warnings that if the update was not done that there could be warranty issues down the raod.
The update was automatically done when I had it in for the oil service and some other matters in mid-December. Crowfoot said that the update was cumulative, meaning all other software updates would be done at the same time if some earlier ones had been missed since the last service.
I have not noitced a horsepower loss and in fact the engine operation seems to have improved a bit, I have noticed a little less "stumbling" than before.
Because they are cumulative, and because they appear do softweare updates automatically at Crowfoot, I think you cannot avoid the update even if you wanted to.