Spin luff is definitely too long. WAY too long.

You can correct this by lowering the tip of the spinnaker pole by shortening the rigging. We went through a period where the spinnaker luffs got longer on the Nacra 20 and we ended up with so much prebend on the poles that they would break if you looked at them wrong. I ended up rigging the middle part of my pole to extend it lower at the forstay bridles so it wouldn't have to have so much prebend.

If leaving the spinnaker sheeted through the puffs helped with boat control, you are probably over sheeted...which is not fast. That said, it is good to keep the spinnaker properly trimmed through the puffs and not ease it to depower it unless "all is lost!". Fix the spin luff issue and you'll find the boat will be very different downwind.