Nick
With the stock setup the shackle sits with the lower pin aft of the upper. The stock leaves are flat or nearly so, and with flex in either direction the leaves actually get shorter as they arch. This means the lower pin on the shackle will only travel forward, and it's free to do so. With longer more arched springs the spring gets longer upon compression, as the arch flattens. This forces the shackle's lower pin to swing rearward. At stock length the shackle doesn't have far to travel before it hits the frame, so the energy of the bump that caused the compression goes right into the frame and on into you. With a longer shackle, the lower pin can swing rearward freely, and the springs will absorb the bumps.
OME also makes a heavy duty spring pack that is rumored to net about 3". Add a 1.5" shackle to that, and you'll be sittin' right about 4.5". That's the option I've been thinking about, though the Alcan springs are looking good as well.