Hi Rob,
Congratulations on getting a spin. That's one big spin! I've got a fairly recent Goodall spin (17.5) that is flatter than older cuts. The flatter cut does allow you to reach higher without curling the luff. So you may have a fuller cut. Also, there's a luff tension line you could play with. In very light winds I can sheet it fairly flat, tighten the luff, and go higher than a beam reach in very light zyphers (barely rippling the water).

REGARDLESS, there is definitely no way to reach higher than beam reach in 12-20knots without capsizing (on side; there's no pitchpole tendency w/spin)...unless crew weighs 500lbs. For perspective, this weekend I was flying a hull in 5knts with my 17.5m2 spin (cat rigged, solo) on a beam reach and could keep the hull up bearing off for a ways. When I've sailed the spin in a breeze I've always had to keep it deep to stay upright. It screams too. Steve's rig on the mosquito sounds like it's a lot different than the Taipan, and that may factor in.