In this paper, test data is combined with results from two different Computational Fluid Dynamics (CFD) models to investigate the leakage performance of leaf seals. Experimental data is gathered for centric rotor position using a rotating test rig at various rotational speeds, inlet pressures and preswirl velocities. The test results are compared to brush and labyrinth seal leakage data from previous studies and reveal elevated leakage rates of the leaf seal. As the tested leaf seals are subject to thermal leaf deformation from welding during the manufacturing process, the influence of geometry variations within the leaf pack on leakage perfomance is investigated with the help of numerical simulations. Both a fully resolved leaf model and a modeling approach based on porous media is used. The CFD models are validated based on pressure measurements within the up- and downstream coverplate gaps at three different radii. Both CFD models show good agreement with test data for different inlet parameters. A variation of cold clearance shows moderate influence on leakage and small clearances can be brought into context with hydrodynamic lift-up indicated by experimental leakage data. Much higher sensitivity on leakage mass flow is predicted for variations in leaf spacing at the leaf root and leaf tip. The latter is discussed as an explanation for the measured leakage of the test seal with its manufacturing variations, while the first quantitatively shows optimization potential at the design stage of leaf seals.