These aileron fences do not help performance. They may help with some other issues:
1) most gliders require counter-aileron while circling. The fences prevent ailerons from being sucked to the upper side when the aileron is deflected. The suction is applied by beginning airflow seperation at high angles of attack. This issue exists with some older gliders (e.g. LS1, Libelle) when getting close to stall while circling.
2) slight improvement of aileron authority
3) improve airflow on wing/fuselage intersection on flapped gliders