Has anyone added just an aftermarket rear sway bar?
 

My friend, with his S13 just added some cheapo large rear sway bar, leaving the front standard i believe. The intention was to increase horizontal movement in the rear, and kill vertical (roll), for the car to become much more, er, oversteer-y. And it worked. Turn in is much more neutral now and you can feel the rear want to controllably rotate at higher speed.
I'm in the process of picking out a coilover kit for my 370z and thought what if I did the same, and just bought a rear bar only for the Zed.
Has anyone upgraded only their rear sway bar?
or do you have to do both pretty much

Noop, The norm is to upgrade the front swaybar only with a Hotchkis front swaybar.

Agreed. Typically those who want a Z more ready/willing to rotate go with a square tire set up with a stiff front swaybar to prevent snap oversteer. A bar stiffer than stock in the rear tends to lead to a *very* twitchy and unpredictable car.

You want a tail happy Z? Install a Hotchkis rear bar, and set it to full stiff. Turn off the VDC, and do the YAW sensor mod, and a tight LSD. You'll be drifting right out of the driveway. :driving:

I've got Stillen front & rear and she carves corners like she's on rails...also on kwv3's...20mm spacers all around

The answer is stiffer front bar and factory rear bar. If you need more rotation from the rear use the gas pedal. The Z has enough power rotate the rear mid corner. For initial turn-in get some FUCA's and dial in -2.5° to -3.0° of camber.

Man!...............you take all the fun out of my post. :driving: :rofl2:

This is my setup

Thank you guys! So why only the front bar? Wouldn't that result in a super stiff and flat front end and an unbalanced sloppy rolling rear?

Short answer. The front needs more grip. The Hotchkis bar is one way of doing it.