I too have found that the 14"+ pads on my 2015 F350 stop better than the 12" pads on my '99 F350. :)
But if you did not have mushy brakes before, and all they did was rotors and pads, the air in the system and the hoses are the same before and after. My guess is they feel spongy now because the replacement pads have a lower coefficient of friction, requiring a heavier foot and higher brake pressure, which will flex everything more.
The heat generated in stopping the truck will be independent of the pad chosen (1st law of thermo).
At least you confirmed why it is you always turn your own wrenches!
