One thing I noticed when I swapped to rear disc with my dually was the softer pedal. It stops fine, actually a little better IMHO. The kit I got was from
www.diy4x.com, it came with a 1/4" spacer that fits between the caliper piston and the pad. This gave a lot of pedal firmness back after the swap. At first I had installed the caliper without the spacer, but the pedal would travel almost to its limit before stopping the truck. Now it is very comparable to the factory setup after I installed the spacer. I don't know if your kit came with that, but it might be worth a shot.