My experience (limited to Ts) when going downwind is

- if there is very light wind it does not pay off to get one hull out of the water at all costs. Going deeper with both hulls in water will be only a little bit slower but definitely increases your VMG.

- in medium wind conditions it depends on the situation. If there are no other boats close to you, you may gain a small advantage by slightly favouring depth over speed. If other boats are close you need speed and power to prevent them from overtaking.
In any case it seems to me that the way how you go downwind (both on the hull, one trapped out, both on the wire) does not make a really big difference with respect to VMG.

Ceitzi