Heck, if you just want to level your truck and don't really wheel it a lot I'll trade you my Bilstein 5100 lifting shocks for your Icons!!!
But really, your gas mileage may go down a bit, but it will be hard to properly calculate because your speedometer and odometer will be running slightly slower. You can fix this by getting a Truespeed. I just deal with it for now...
You could use the diff drop, but it probably isn't necessary. They don't do a whole lot and they prevent you from running a good skid plate unless it's modified. They do bring down the CV angle slightly....but it isn't much.
It is likely that your inner CV's will start to spit grease. This happens a lot. If they aren't torn, just take the stock clamp off and use two hose clamps per side (the screws on opposite ends to counterweight each other) and tighten them down. This holds the boot on slightly tighter. Search for the CV boot mod. This is what they are talking about.