Coil spring spacers

[guruerror]

I'm contemplating rear coil spring spacers.

I currently have the Cognito leveling kit with the Bilstein shocks. The rig sets level but I'd like to put another 1.5-2" between the truck and the ground. I figured the quickest way to do this is to crank the torsion keys and add some spacers.

Questions:
Polyurethane vs. Aluminum spacers...which is the way to go?
How hard is it to install spacers?
Any issues/concerns when using spacers?