Dave,I'd be very inclined to believe that you'll see some benefit.My original shell ( pre-goats ) had a step for the 3rd brake light and another for a vertical rear backlight.It was still good for 13%.
Both Ferrari and Porsche have used the multi-step backlight louvered cover.
And Ford's patented bi-wing spoiler ( Probe-III,Sierra,Merkur XR4Ti ) is kind of a cruder version,but with good effect.
Your steps should provide for re-attachment and creation of locked-vortices which will help smooth the outer flow.
And thinking about Frank's comment,if your cab-to-bed tailgate ratio is 'fast',the rear shelf would help shape the vortex closer to the 'template' curve.
I'd do it.
