Have you got it yet?
Nope. They told me one week to the states and two weks for springs and valving customization. So I figure it is about a week out, as is the front suspension.
Nice.
Does it make sense to just change the rear shock and not the front ones?
It will provide some improvement, but the two should be coordinated to work well together. In my own opinion, the front was worse than the rear and if I were only going to do one, I would have done the front. Neither suspension is "adjustable" other than rear pre-load, so making any improvements will require dismantling and tinkering. The front is easier to tinker with. One can change oil weight and adjust compression and rebound damping by altering the size of the ports in the damper rods. From my imperfect knowledge of suspensions, it needs less compression damping, more rebound damping and more progressive spring rates. The ability to adjust damping or to provide different low and high speed response would require more sophisticated valving. A tinkerer could put in heavier fork oil to increase the rebound damping and drill out the compression ports to decrease compression damping. Then, though the springs could not be made more progressive, one could adjust the sag by spacer adjustments as necessary.