If your base weight is near 16 right now, you've picked most of the low-hanging fruit already. Pushing down to 12 lbs will mean a combination of spending a good chunk of money on the lightest, best gear and going purely Spartan when it comes to luxuries. I would say you're on the right track looking at a WM Megalite. It is pricey, but you're getting the best solution that exists, so it is worth some money.

As Jimshaw indicated, you can't get around the weight of 6L of water. And critical gear is just that -- critical. There comes a point where removing items or cutting back weight can start to encroach on bad judgement or plain foolishness.

You might get away with a lightly built pack that can't comfortably handle loads above 25 lbs -- if you decide that, for a time, you will just be very uncomfortable until you drink up some of the water. Just be sure the extra load won't overstress either you or the pack to the point where something gives way!