Adjusting the traveller a foot or not would depend on the type of boat in my thinking. What angle is the boat sailing for best VMG, and how can the main be set to complement the spi. The speed and the telltales will give the answer. If you know the best angle for the conditions, it should be relatively easy to trim the main to the spi by looking at the telltales and the feel in the boat.

Saying that downwind power is everything is a bit.. rough? A kite with a lot of draft will give more power, and drag. It was when we moved to kites with less draft we really got speed. In line with the same thinking, not letting the mast all out will in my humble experience give more speed when sailing downwind with spi, but it is harder on the mast so be careful.