It is true. The flow is lost from the valve sinking farther into the bowl, not up from the chamber. If you are removing material from the seat you are sinking the valve, even if it is an iota. My comments were aimed at a machinist who wanted to do a valve job no matter what, any excuse will do, to get your money. I agree with lapping and always lap a couple of each valves randomly after a valve job, since it will give you a visual of how good the seal is and the width of the seats. You give an example of how a valve job should be done and will lose almost no power, and gain a lot if the valves were not sealing. It all depends on the frequency of the valve job and how deep the seats are sunk each time.
