VHF Data Broadcast (VDB) currently used by GBAS has been identified as a potential source of cyber-security concerns. The use of an alternative datalink providing the bandwidth for more capable security protocols has therefore been proposed and demonstrated on the basis of the L-band Digital Aeronautical Communication System (LDACS). However, the first demonstration of secure GBAS over LDACS suffered from some performance degradation. This paper provides an improved method for secure GBAS over LDACS on the basis of a rigid performance analysis. Optimized parameters are derived and evaluated. The results point the way for further performance enhancements for even more challenging GBAS scenarios.