If I remember rightly (and I may not since I don't own a DX5e), the built-in mixing on the DX5e limits the elevator channel to around 60% so your robot will seem unusually slow when going forward and back, but will turn left and right at the correct speed. It's not something that's adjustable as far as I'm aware so your best bet would be to leave it turned off and use an external mixer if you want single stick driving. Although watch what external mixer you buy as some of them also have fwd/bck limits.