The boxer motor is balanced very very well, it is perfectly good to run a lightweight crank pulleywith side effects or codes.
You will get lower rpm responsiveness and acceleration will be a little better, I have a kartboy lightweight pulley and have had zero problems in the five months its been on my 2.2.
I'm sure since you discredited the underdrive pulley you understand the problem that piece contains?
How do those 8mm wires work out for you?
welcome by the way...