To my understanding whip tillering would reduce the speed not keep it up, and piking would make them heavier, not lighter. If you feel the bow is overbuilt and could handle the stress you could reduce the width. That should be the easiest way of reducing weight without effect tiller that much. However, if the bow already has set, or is bending far for its length, you probably would have to just thin it down, and re-tiller them completely. Ideally, tiller should be the same, your just ensuring it stays that way as you take wood off. Should be faster than making a bow, but it will require the same carefull attention to tiller as when you made it. Sorry, but I don't think there is an easy way to reduce weight like piking increases it.