I have read a bunch more forum posts about suspension modifications, and I have learned about the importance of shock valving! Thank you to the folks on this forum who explained the importance of it better in other posts, especially this one and especially @Bill Verburg:
https://forums.pelicanparts.com/porsche-911-technical-forum/876392-bilsteins-stock-suspension-hd-digressive.html
It took me a few times to read those graphs and understand what was meant by digressive vs linear or progressive, so I thought back to physics and controls to make it all make sense to me.
Force = mass * acceleration
In our case, the mass is the unsprung mass of the wheel, brake caliper & rotor, wheel bearing, and suspension arm(s) holding the wheel to the car. The two items acting on our unsprung mass are the damper and the spring (torsion bar). They exert force in different ways.
Force of spring = -k * x where k is a constant inherent to the spring and x is the position measured away from resting; the further the spring goes away from its resting, neutral position, the more force it exerts. This is why mild cornering produces mild roll and aggressive cornering produces aggressive roll.
Force of linear damper = c * v where c is a constant inherent to the damper and v is the velocity of the mass, our wheel. As velocity increases, or as shocks become more abrupt, damping force increases. That is why, on my car with linear dampers, the sharper the bump is the more aggressive the damping reaction is. A little body roll in a corner doesn't incur much damping force, but a sharp impact over a pothole causes lots of force. Linear shocks can be overdamped, where they produce more force than necessary to bring the mass to its resting place to the extent that it takes longer for the mass to come to rest. Or, they can be underdamped, where they produce so little force that the mass bobs back and forth for a long time before it comes to rest.
Check out this cool video showing how changing those variables impacts motion:
https://www.youtube.com/watch?v=PwlntnWtqJc
Now here's why digressive damping rules for sports cars. With digressive dampers, the "C" constant in the equation becomes velocity dependent: at high velocities, a digressive damper will exert less force and the "C" constant will be lower. At low velocities, it will exert more force and the "C" constant will be higher. The main example of low velocity suspension movement is body roll; we want less roll for better traction, so high damping force to counteract roll is good. The main example of high velocity suspension movement is bumps; we want optimal damping force, not too much and not too little, over bumps to bring the suspension to rest as quickly as possible and, therefore, regain grip as quickly as possible. A mass is "critically damped" when, for a given spring and a given displacement, the time it takes for the mass to come to rest is as short as possible. It doesn't rebound back and forth, but it doesn't resist the motion so strongly that it keeps the mass away from equilibrium.
According to the experts at Elephant Racing and 3R Racing/Performance, my linear dampers are the primary cause of the stiffness I'm experiencing over potholes and expansion joints; I just spoke with Chuck of Elephant Racing and Brian at 3R Racing & Performance (the local Elephant Racing installer). They both think the 21mm front and 27mm rear bars are not stiff at all, and that going to digressive valves is the way. The only question now is whether re-valving the Bilstein HDs I bought three years ago or buying new Von shocks is cheaper. The Vons have spherical joints on the bottom, which is better, but they are shorter than the Bilsteins, which means they're better suited for lowered cars. I want to go higher on ride height just as a way of avoiding scraping; too many steep driveways in the mountains of Colorado.
So, more research to be done, but definitely going for digressive shocks and definitely keeping my torsion bars and poly bronze bushings. For now. More to come, I will be implementing one form or another of these digressive shocks soon! I'm definitely going to get some video on my new GoPro of the wheels over certain bumps before and after this change, I may get wild and crazy and see if I can't find a sensor that I can stick on the suspension arms to log movement and measure the reactions before and after.
Science! Nerd-dom! Performance! Wallet? Better go make more money...