Thanks for the replies, guys.
I don't want this to come across as argumentative (I'm the one asking for help, here!), but I do want to clarify the poly bushing part. I don't want to prove you wrong, but I want you to prove me wrong.
I don't agree that we have to replace all
poly bushings because in some places they should work. For example, a poly bushing on the front, lower control arms does not provide any opportunity for binding because there's no situation in which it should be allowed to twist. Also, the rear LCAs should only bind if there is a tilting motion of the rear axle (e.g. one tire drops into a pot hole while the other is on even pavement). That binding can be alleviated by the heim joint on the front allowing the LCA to turn/twist. If both the front and rear joints were poly in that case, you'd have a problem. The bushings on the top of the pumpkin that the rear, upper control arms attach to clearly have the propensity to bind and should only be rubber (which I've fixed).
As for the springs, it appears that the SC&C springs have a higher spring rate in the front (535 vs 358) and about the same in the rear (135-150 vs 158) compared to what I already have. My guess is Jeff is right that my suspension is "at the top of the arc" which is part of my problem. Perhaps the SC&C springs are stiffer but shorter and would allow more travel. I think the ride of the car supports this theory, because I don't feel a jarring bump
when going over a bump in the road so much as I feel a bang when going over a drop in the road. If I hit line of uneven pavement when the back of the car drops down (e.g. going from concrete to asphalt where the asphalt is slightly lower), it feels like the suspension completely unloads (especially in the back), the tires come off the ground, and then I'm jarred when the tires hit the ground again.
I'm eager to hear any additional input (suggestions of spring/shock combos, troubleshooting ideas, theories on poly bushings, etc, etc). I'm an analytical guy, so get technical.