Good leadership makes a huge difference - whether that be big or small. I've seen it myself - in particular it's amazing much of a difference good leadership can make at a huge company.

However at the same time it can also be true that some leaders are either poor or make no difference - but as long as they don't have to react to external change ( or only do so as part of a herd ) they can coast quite happily for years while 'earning' huge rewards.

Leadership is only really tested when the right course is a different one from either the past or your peers.