I'll preface my comment by saying that my intention isn't to be insulting in the least....your power is likely diminished because your hips are simply too weak to pull off proper technique. If you are going to commit to pulling sumo, my advice would be to start with picture perfect form and make every rep look the same, regardless of weight. If you have less than perfect form, you're setting yourself up to hit a plateau in your progress earlier rather than later. If you must drop the weight in the beginning, do so. It'll pay off in the long run.
Personally, I played with going toward sumo. Through a combination of my leverages and desire to pull heavy weight, I opted not to start over again at the beginning. If if I were seriously going to pull sumo, I'd start at 135 lbs and ensure that every subsequent rep with heavier poundages look the same. Knees shouldn't be over the bar, shoulders should be above or slightly behind the bar. Chad Aich's DVD gives some good pointers on pulling sumo.