Downwind VMG

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So I'm thinking about racing Sylvia at the LLR this year (Leech Lake Regatta).  Since I've owned the boat I have yet to really figure out the best downwind performance out of her.  Wing on wing is a bit sketchy in anything but light to moderate consistent wind, and she doesn't sail very deep before the genoa is blanketed by the main and goes limp.  In heavier wind I've found she handles nice with the main down and the genoa all the way out, with the added angle to dead downwind gained from that it makes up for the slight loss in speed.  But this method only seems effective when the wind is over mid teens.

Has anyone done any "testing" to see what the most effective downwind sail setup is?  Maybe I need a spinnaker to be competitive.