Thanks for that, k2e.., what I mean by balance in the Subaru motor is the flat design pushes thrust back to the center of the motor, the reason why we don't have dampers or balancers...honda snapped to that with the k series motor, all of the pulleys on that design are lightweight. I built a few b18c's, and I have used l/w pulleys on one...it blew up on the track (but I had a good quarter time the first run)
Of course your absolutely correct about torsional vibration is a by product high horsepower motors. I'm saying that vibration is less prevalent in a boxer motor.
I know very little about the d series motors, when I had my civics I took the d16's out of the car tied it to my boat and used them as an anchors or counterweights.