KE = 0.5mv^2, so the second stage would only have 16x the kinetic energy if the two stages had the same mass.

The dry mass of the first stage is approximately 25,600kg. At 2km/s, its kinetic energy is 51.2 terajoules. The dry mass of the second stage is approximately 3,900kg. At 8km/s, its kinetic energy is 125 terajoules.

So the KE of the second stage is actually about ~2.4x that of the first stage.

Here is were I found the masses of the stages: http://spaceflight101.com/spacerockets/falcon-9-v1-1-f9r/