if you hit the jump/bump with more speed a couple of things will happen:
you won't have to jump up with your legs as much because the speed will propel you off of the jump
you'll be going a little bigger, and will have a little more time in the air to turn the board around - and see your landing
you may have time to grab your board (or at least try) which will make you suck up your knees in the air - giving you more time
don't worry about "needing to get smaller and loading your twist" - don't need that much for 180's... instead try to concentrate on counter-rotating your body as you approach the jump, and as you are riding off of the takeoff begin your rotation with your upper body - shoulders, neck, head will initiate the spin before you even leave the lip, and your lower body will just come around.
also, practice jumping 180's and 360's in a snowboarding stance out on the lawn with no board on - it will teach you to keep your eyes open, counter rotation before the jump, etc.