That's interesting about the effect of luff tension on the spinnaker shape, and I guess it makes sense.
With the spinnakers Lindsay makes we usually aim to have enough slack in the luff to be able to grab a handfull and turn it 180 degrees (probably about 200mm of slack). This is the way the spinnaker is designed to set so we don't really play around with it. I think you have a tighter luff than this from the photos - and the head does not seem to even go all the way to the halyard pulley. It looks like your pulley could be set lower. Hard to say from a photo taken in no wind though.