I agree with Jake...tensioning the spin halyard adds draft/shape to the luff of the spin....while easing flattens it. You can test this while ashore with the sail raised and sighting along the luff under different tensions. You can get more tension by putting more pre-bend in the pole end if needed.
Keep the spin loose-sheeted in the light...just enough to keep her inflated...luff should curl and re-fill periodically. Over sheeting is death!
Sail as high as needed to get moving, but don't go crazy trying for more speed or hull flying until true winds are beter (typically 6-8 kts TWS is the transition for the Tornado)
Keep all weight as forward as possible...keeps the transoms out and boat glides silently.
I've found it a big benefit for one person to steer & run the spinny at the same time. Makes coordinating moment by moment adjustments so much better...lees over-correcting on both the helm and sheeting with each wave/puff.
In the real drifters, consider popping a rudder and a board or boards up...but like others have said, take current affects into consideration.
Did you have you spin halyard tight? If so, ease off 6" or so. If you have a tack line, try easing that as well or instead. You may want to check your main and put on more downhaul and maybe even less mainsheet. If your boards were up and you were sailing high you may have been crabbing sideways.
I don't know about the melges, that is a big spin and in 4-6 it was helping them. Not unusually with the A Classers.
Loosen the spin luff? That allows the kite to rotate to leeward which is not what you want if you are trying to go deep. Loosen the luff if you want to go high. It is contrary to intuition, but you should tighten the luff to bring the kite further to windward to keep it in the flow for deep downwind sailing.
