The requirements for a CAT-II/III capable GBAS (the so called GBAS Approach Service Type (GAST) D) are derived from the definition of a safe landing. The respective performance requirements are given in terms of touchdown performance of the aircraft which has two main influencing parameters: the flight technical error (FTE) and the navigation system error (NSE). In the process of deriving and standardizing the GBAS requirements a fixed value for the FTE is assumed. In this paper we show potential benefits from using the deviations from the nominal approach path to assess the FTE performance during an approach instead of using conservative assumptions. Depending on the prevailing wind conditions, the FTE performance is typically better than the value which is derived in the certification of an aircraft. This opens the potential to either improve availability of the GBAS service or optimize the landing with respect to runway capacity or risk minimization for a runway overrun.