I spent a weekend of beating on it at the auto-x taking 1G corners from 30-70 mph and had no issues with clunking.

I agree...Check your endlink connections. Most ball style endlinks need to have a wrench on the back side to keep the ball joint from turning when tightening the outer bolt. Failing to do this will leave that connection loose and consequently produce some noise as the bolt moves in the swaybar hole..

Sorry to bring this thread from the dead.... But I have BC Coilovers and I experience this Clunk as well. I do not have after market sways.

It sounds as if the spring is popping in and out in the rear. However I have my Coils with the 230MM Spring and I'm lowered about 1.25. It seems to go away once suspension or car warms up. I Have no clue what it is, but any ideas/input would be great.

That's exactly what it is. You need a little bit of pre-load on the spring when the suspension is fully extended. Jack the back of the car up but let the rear suspension hang, shake the spring and check that it is not loose. If it is then you either need to shrink the shock body to match the spring or extend the spring a little using the adjuster.