I personally would never use Shimano pads if I could help it (nor cables, chains etc), I think they profiteer on those small parts. People like Fibrax and Clarks have to offer good quality and they do. I think Gus may be right that if the ceramic surface of the rims is worn, the effectiveness of ceramic pads is diminished. Although in theory we're just coming up to the conditions where you need them most.
I don't like the sound of lack of power though, are you sure there isn't flex in the system somewhere? e.g., XTR sounds good, but the power in levers comes from how sound the bushings are - brand new Deore levers are more powerful than worn XTR. I've seen systems where you could apply the brake and then just keep squeezing the lever further. That's when you get no power, it could be flex in the rims, pads out of alignment, stretchy cable, worn bushes in the lever, worn bushes in the brakes, all those sorts of things. All the cash gets spent on sexy brake arms, but I don't believe that even the cheapest brake arms flex, it's the unseen things that flex.
I always use the wider washers, but I don't think it gives you any more leverage. Leverage is maximised by setting the pads very close to the rims. So ensure the wheels are dead true, and the brakes are dead centred and then tighten the cable with the barrel adjuster after putting the wheel on.
If you have the right pads, eliminate any possible flex and set the pads close to the rims, you should have plenty of power.
