This success was unexpected. That means that they can recover a stage from a higher speed than previously thought. Does that mean that the recovery of the second stage from LEO may actually be feasible?
Second stage recovery is a whole different beast. Recovering the first stage on a flight like this is a delicate dance because margins are so small, so everything has to go exactly right. The previous attempt (SES-9) didn't quite go exactly right, and made a big boom, perhaps because the engine burns were slightly mistimed.
Recovering the second stage is difficult because it's coming in from a much higher speed. The first stage came in doing about 2km/s. The second stage would come in doing 8km/s or more. That means 16x more kinetic energy to deal with and 64x more heating. You need a proper heat shield, not just clever engine burns. There's much less extra margin to play with as well. The second stage does most of the work but is much lighter, and one pound on the second stage is worth ten pounds on the first stage.
SpaceX definitely believes second stage recovery to be feasible at some point, but not on the current Falcon 9.
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.
The second stage has a smaller ballistic coefficient than the first one, though. Their cross-sectional areas are the same, but an empty second stage has much less mass. If it could survive entry (and that's a big if), it's terminal velocity would be smaller.
