From what I've read about how critical these are this sentance pretty much sums it up I suspect.

Someone in another post was describing the shape of the 8035 and the "old" S7037 and how on a 10 inch chord the differences between the two are less than 30 thou or so and how important that 30 thou was if you expect to see the advantages in the new airfoil. Covering sag would account for much more difference than that so any advantages of one over the other just go down the flusher if you have any open bay style design at all.

D tube construction for such wings was also blasted by non other than Micheal Selig in Soartech 8. The transition at the rear edge of the D tube between the ribs wreaks havoc on the airflow in you're serious about your sailplane flying. Extending the sheeting on the upper surface back to about 60to 70% of the chord helps a lot to aid the airflow. But on shapes where the trailing edge cusp or local droop is pronounced, as in something like the S1223, the only way to get the advertised performance out of such airfoils is total sheeting or vac bagging.