The amplification is not additive. You'll still be using the factory system's amp so it'll be fairly close in loudness to the stock head unit. However, you'll likely have more control with the Kenwood so the apparent loudness may be more depending upon how you tweak your settings. I did more or less the same thing with my JK. I kept the factory Infinity amp and wiring, but upgraded the speakers, baffling in the speaker soffits and put in a Kenwood Nav head unit. The sound was so much better quality, that I didn't need to turn it up as loud as the stock system.
2010 Brilliant Black Crystal Pearl Coat Grand Cherokee S-Limited with various bits 'n pieces... broken in but not even finished!