Question is wether the problem with going downwind with spi is staying in position or manouvering. Doing a quick risk evaluation manouvering (gybing, coming in, going out etc) is time consuming one-up and prone to going awry. Only way I can see to handle that is time in the boat and specific training for good routines (doing drills).

The yellow spi in the pic sure was loaded. Look at the seams and how the cloth is stretching. A spi built from more panels would stretch less. Could be important, but not if the driver is going to loose 10 boatlengths gybing