Torsion keys will not address the problem with downtravel. Your lower arms will be out near full downtravel when you lift. Only spindles will still give you a decent cycling range in both directions.
I've had both Doetsch spindles and Fabtech on this truck and the Fabtech lift it higher and don't require much t-bar tightening to get a good lift.
The main difference is when you look under the truck. Spindle lifts, unless the people are trying for even more lift, have the lower arms more parallel to the ground. T-bar adjusting, or using keys (same thing really) causes the arms to be down-angled.
You'll be much, much better off with spindles.