There is a test in Planche Mag (FR) this month. I only had a glance at the conclusion. It says that the Shox increases comfort and possibly speed for intermediate sailors, but the difference is not significant for advanced ones. Various sail/board combinations are tested. I will post more details tonight.
When the tendon joint was introduced, it was supposed to be faster because stiffer. Mostly a matter of feeling, maybe.
I wonder if an old school joint would not do 80% of the job.