When you are talking about and comparing 60' times, you do have to be cognizant of the HP that the car is putting down. Cutting a good 60' takes traction and HP. Now, if you ask the guys on here who have gone in the 1.30s, ie, JnJ, Dr Hunt, Z-Gad, Turbo Meister, myself and I apologize if I omitted someone else, you will probably get all different answers on what works for them. The one thing in common is that they are (were) all running automatics and slicks/ET Streets. Of all those Z-cars listed, mine probably had the least HP but I had the C4 IRS and really worked hard on perfecting the launch to make up for the "low" HP.
With both the Z IRS and C4, my focus was on quick rebound and slow compression in the front and the opposite in the rear. This gives quick weight transfer and does not unload the rear tires too soon. Only way to do that is with adjustable shocks and I used F&R Illuminas on the Z and a coilover/adjustable shocks in the rear on the C4. Lots of debate on squat and I will not start that debate again. I looked at squat as weight transfer and took advantage of it. I used to launch the Z with the C4 IRS at 4900 off the t-brake, drag the bumper and cut mid-1.30s. 4900 does not sound like much until you realize I shifted at 5700 As for stifness, I ran 180# springs in the rear and on the RX7 with the stock IRS, I run 120# QA1s.
It also depends on whether you are running off a t-brake or footbrake. The t-brake will induce dramatic squat soon as the car moves and the footbrake will load the rear suspension as you are footbraking. Since I footbrake the RX7 I run soft springs. Now understand that is what worked for me. Some guys go to extremes to completely eliminate squat and i guess that also works for them. Ironically, if you are running a manual you have your hands full but I would imagine you set up the suspension as would if you were using a t-brake.