Sorry, but if you have calibrated at 100% and increase to 140% you won't notice any difference other than the speedo might complain (some are intelligent enough to know what the end points are and if the radio goes outside those points).
It works like this, your radio generates pulses, 1.5ms pulse with is neutral, 1ms is full brake and 2ms is full throttle and as you move the stick the pulse width changes in between these values.
If you go to 140% you will still generate 1.5ms wide pulse at neutral but the smallest and largest pulses will now be 0.8ms and 2.2ms respectively.
You only really need to 'overdrive' the steering servo to get more travel (sometimes) and it only works with digital servos.
Most speedos won't calibrate if the pulse width is outside the normal values.
Equally if you reduce the EPA you get less travel (throttle/brake) and as you state that 75% brake isn't enough then you need to find the sweet spot on your EPA, as you've told the radio not to produce full width brake pulses.
Apologies if this is teaching you to suck eggs
Ian