Is this the BC video you followed for setting the preload?
https://www.youtube.com/watch?v=HlqOA35NmAw
*This is a POOR method. You really need to measure the height from the top of the spring perch to the bottom of the spring perch and make sure the right and left sides are even -before fine tuning on a set of scales to corner balance the car. The method above can lead to significant differences right to left because the 'snug' setting is extremely subjective and could be 2-3 turns different from side to side. I'm surprised BC posted this video. The NSX scales very well and i've had crossweights withing 0.2% by setting the right and left sides the same length, and 2-3 turn differences could easily swing the cross weights beyond 0.2%
For the NSX, BC's appear to have very little total shock travel which can bottom out in both compression as well as rebound (droop travel, shock extension). It also appears the BC coilovers do not have that much rebound travel or a helper/'tender' spring.
If you're maxed out in adjustment to raise the ride height with the lower collar, you can raise the spring perch even higher (adding preload) to raise the ride height of the car. This does 'pre-loads' the spring, so if you have a 10Kg spring (560lb-in) and raise the spring perch 1/4", you'll actually raise the car ~3/8" (due to the car's motion ratio) and add ~140lbs to the initial seat pressure of the spring.
To simplify: raising the front 1/4" at the spring perch will raise the car more than 1/4", make the initial ride slightly stiffer, and reduce droop travel which may hurt ride quality over big bumps. But if you're bottoming out, it may improve the compression side of the stroke.
I think the main issue you have is that you're running 18/19" wheels (which hurts ride quality) with a lower quality shock, with very little shock travel (which REALLY hurts ride quality). All of these compounding on top of eachother is not a good thing.
KW's and many other coilover brands have a proper amount of droop travel, they run helper/tender springs, and do not suffer from bottoming out in compression or droop travel. Keep in mind that OEM suspensions have a TON of droop travel -which is important for ride quality, especially with softer springs.
Be aware that lowering springs with not enough increase in spring rate could lead to bottoming out the shock in to the bump stop (which is REALLY bad for ride quality). I've driven far more cars with lowering springs that rode worse because they bottomed out in the bump rubber than ones that were engineered properly with enough spring rate to keep them out of the bump rubber.
0.02
