The '88 through '91 Z51/Z07 springs were all the same rate (and aggressive). For the '89 to '91's the car can't have FX3. Even though Z51 was required for FX3, those have the same spring as the base susp.
I have an '89 Z51 non-FX-3, and it IS stiff. Haven't auto-x'd yet, but I plan to get into that.