Trey, it's good to see that you really like the Zeal B-6, as these were the coil-overs I was thinking of getting. However I noticed that King Motorsports for example are using the Moton shocks with separate reservoirs on their racing ITR, and all high end racing shocks tend to have independant bump and rebound adjustement. In particular most of the racing shocks from Koni like the 2812 and 2816 ($995 each shock) provide independant adjustment for bump and rebound, as do Penske's, Dynamic Suspensions, Quantum and other very expensive shocks. I spoke to Benoit Theedge at Watkins Glen and he upgraded to Dynamic Supensions shocks on his Grand-Am NSX, and P.D. Cunninham upgraded to Moton shocks for his NSX. Therefore there are obviously a lot of racers who believe the extra adjustability is worth paying for.
The bump damping controls the unsprung weight, and thus needs to be adjusted for the weight of the tires/wheels/suspension, while the rebound damping controls the release of energy stored in the compressed spring, as well as the speed at which the car transitions to roll in cornering. Rebound can be used to control the balance front to rear in terms of corner entry understeer or oversteer. Thus bump gets adjusted for one thing, and rebound gets adjusted for another. Rebound is changed based on spring rate and balance adjustment. Since the springs are selected based on the mass of the car to get a particular resonant frequency, one could say that bump is adjusted for unsprung weight control while rebound is for sprung weight control. Low speed bump and rebound adjust how the car feels when transitioning into roll, while high speed bump and rebound adjust response to small road irregularities and bigger ones like curbs where the shock piston is moving really fast, controlling how well the tires stays stuck to the track on bumps. Low speed to high speed damping ratios need shock revalving, unless you get some really trick 3 or 4 way adjustable shocks.
The benefits of Tein vs. Zeal vs. others have been discussed extensively on many boards, but I have not seen any discussion relating to higher-end coil-overs more in the tune with what the some of the racing world seems to be using.
When I noticed that Truechoice offered double adjustable shocks as part of their coil-overs for both Phase III (~$1880) and IV (~$2020), I wondered who else had some in the $2k range, and if anybody had had good results with them. The Advance-Design ones are about $1600 for a set (plus springs and seats I think) and they have the two adjustment wheels at one end like a true Koni racing shock, appearing to make it very easy to adjust one or the other by opening the hood or hatch. I wanted to know if anyone had ever tried either the Truechoice or the Advance and what they thought.