you want to target 325-350lbs/in range if you want a soft ride, run
no higher than 35psi and use minimum 45 profile tires (ie: 18's)

 

I assume your mentioned spring rate is for the rear, correct? I am running 50 profile, 17" Goodyear GS-D3s all around at 30 psi already. It's beginning to look like I'll only have to change to softer springs all around, like the Tanabe NF210s that were recommended above, to get the ride I'm after. But then, maybe I could just have the rear springs from the coupe with 17" wheels substituted for the ones that came with my original 19" wheels. That's the difference between 427 and 342 lbs. I'm sure that would make quite a difference.

Are the shocks/struts the same on coupes with or without Sport suspension? Thanks for all your help everyone.

 

actually in this case either front or rear. but the ride stiffness feel
comes from the rear rates (unless your car's lowered too far and the
front is hitting the bumpstops over large bumps).

running 342 will definitely make your car feel softer.