MaysEffect

The Z is not the only exception. Almost every car with a greater front weight bias has a larger front bar. The wrong information has been spread over the past decade that the rear bar should be enlarged for a plethora of wrong reasons. In reality the only reason the bars should be increased is if you have a significant increase in torque or increase in weight and or spring rate.

Most cars understeer from either having too much weight on the nose, or not enough at speed, this is why a 50/50 weight balance is ideal or slightly rearward (48f/52r). Under braking and lateral forces, weight shifts forward and outward increasing load on the front tires. Too much load and you overload the tires and lose traction, too little and the tires won't grip. This balancing act is the most critical aspect of design.

The Z has more weight on the front axle, no where near the levels of a Subaru or VW. But combined with smaller tires, you get a loss of traction from tire overload. There isn't anything funky about the suspension design or alignment that would cause additional understeer.

Increasing sway bar size is rather compromising unless you're increasing the spring rate or the amount of lateral load (increased g forces from bigger, stickier tires). Adding a larger bar with the same factory tires will just increase the load on the outter tire and reduce the contact patch on the inner tire. This is no bueno, thus the plethora of complaints of understeer.

Adding a larger front bar will reduce roll as intended with increased lateral load or additional weight (downforce). Increasing the rear bar causes a cross weighting effect that will transfer additional load to the inner front wheel, sharpening steering response and tire grip. The downside being increased instability and increased load on the outter rear tire. The trade offs are vast if you aren't actually increasing tire grip and load capacity.