Actually, setting the D-Pad to attack is pointless.
Having just spent 20 minutes testing mash-outs, the input that seems the have the largest effect is the control stick, so rotating the control stick as quickly as possible is, as far as I can tell, the fastest single way of mashing.
I tried combining this control stick rotation with other things to see if it increased the speed of mashing out, control stick + C-stick/R+L/D-Pad as attack didn't seem to make any difference, however I was able to mash out quickest by rotation the control stick as quickly as possible whilst mashing the A/B/X/Y buttons with the 3 middle fingers on my other hand.
The D-pad on attack was the slowest method I tried of mashing out, even mashing R and L was faster.