I believe one of the more important benefits of the wingsail WITH slots is that it can perform optimally at larger angles of attack and thus make use of high lift coefficients without stalling or seeing high drag coefficients. This is something a soft sail is less suited for.

Now this won't help you too much upwind where the angles of attack are basically around 15 to 20 degrees and you experience very high airflow velocities (200% windspeed). Here you want low lift coefficients and low angles of attack.

However on the downwind legs this all changes. Here you want to have very high lift coefficients as the airflow velocities are so much lower (75% windspeed) and you have to achieve this at angles of attack that are around 60 degrees.

Basically the solid wing has a much larger range of optimal operating conditions then a soft sail.

Wouter