Hmmm correct me if im wrong but isnt 'deadzone' supposed to refer to an area in the centre of the stick where moving it will not register anything in game and increasing/decreasing this value increases or decrease this area??
If my assumption is correct then MAME does not treat deadzone in this manner.
Let me explain, i changed the deadzone to '0.90' fired up Spacer Harrier and harrier move the full width of the screen left, right, down and up (as he should do) but when trying the diagonals harrier didn't hardly move at all from his centre position! As i slowly lowered the deadzone value harrier moved further away from the centre when trying the diagonals with setting '0.1' being the sweet spot where he move just as far in the diagonals as he does for left, right, up and down.
So the MAME deadzone setting only seems to have an effect on digionals inputs and doesnt effect left, right, up or down inputs what so ever!