I still don't see a practical advantage to turning it off. Have you compared moving the throttle from min to max vs restarting the engine? Isn't there a noticeable lag before you get thrust when restarting the engine? I realize that wouldn't always be a disadvantage but it general it seems to outweigh a small difference in drag and torque.
I don't know. I can't understand the practicality of it either, but it is done, and at least one person toggles it in a way that they found advantageous.
There's a definite lag now, like you mention, at least a couple of seconds.
I've also experienced the engine being turned off when the person is going near vertical, when airspeed is nearly gone. It seems to drastically reduce any effects of torque there as well.
On a side note, my film shows the Spit 16 doing 550+ (maybe 560ish) after dropping from 20k to 8k, slightly turning to get a shot on me, and all parts remained on it. I want that.