No doubt that having a longer pole gives you a more vertical component to the drive produced by the spinnaker. But I don't recall seeing any spin boats bury a bow even in the 15-20 mph breeze at Tradewinds on Sunday. It was the least of our worries on the Inter20 and never even presented the remotest of possibilites. I did hear tale of 6.0 driving under but they weren't flying a spinnaker. The inter20 does have a relatively short pole. My 6.0 has a 14' pole and a 28' hoist height and I'm less concerned about burying a bow with the spinnaker than without it. You do have an extreme angle on the luff on the SC17 and I remember remarking how similar the SC17 spinnaker angle looked to that of the Aussie style skiffs (not that there's anything wrong with that).
Would you agree that if your pole is too long that too much of the force generated from the spinnaker would be lost to lift? For example if you take this to the extreme and come up with a spinnaker flying with the luff horizontal - it would not provide any forward drive and would only lift the boat and provide a healing moment. Point being, if the luff is at too much angle, you would not be getting the full benefit of all that sail area. Hence, the less angle at which you attach your spinnaker (and still handle it without driving a bow under) the deeper you can drive it and the faster you'll get to C.
