In most cases less outhaul will give you a bit more power and it will move the draft back in your sail. i use an adjustable OH to get more power and help to get on the plane in marginal winds.
jez,
stick close to the settings recommended on the sail. planing depends on other factors such as where your feet are on the board/wind strength/sail size etc etc.
for sydney's (mostly) gusty wind most people rig 1/2 metre up at least.
crudely - more outhaul = flatter sail = less power (although more control)
