Thanks all. I think the final answer to my question is that changing the dimensions of a spinnaker that will still "fly" is possible but has limitations. On the Corsair the spinnaker could be reduced in size by approx 15% but shaping the girth. This would go from approx 630 ft2 to 535 ft2. This would still be too much for the wind conditions and generate too much boat speed. Another sail combination is the way to go, most likely jib
only (175 ft2, 8kts est) or screacher only (300 ft2, 10-12kts est) with no main or 2nd reef main with a target of 10kts boat speed.
For SailFastTri, I appreciate your cautions but the limiting condition of maintaining 10-12 kts downwind boatspeed were enplaced to prevent stuffing, pitchpoling, rudder
overpowering, uncontrolled rounding, and any other callamities (I think you named them all). Properly weighted, this boat typically will not stuff the bows at less than 15kts boat speed. At 10-12 kts, you have a pretty good safety
factor especially downwind. On this boat having the main up, spinnaker sheet made fast and initial boat speed > 15kts is the common killer. What we are looking at is headsail/spinnaker options to keep a good target boat speed for the conditions. When boat speed goes over 15 kts, the power sheet needs to be in hand.
Thanks all for the discussion,