At REXEBIS, a higher electron current density accelerates charge breeding and allows for increased repetition rate and ion throughput of the REX-ISOLDE system, while also extending the physics reach towards very short-lived radioactive ions. This goal was pursued by introducing a non-adiabatic electron gun. Since then, further breeding characterization experiments have been carried out, focusing on reaching very high charge states for light ions, such that a closed-shell configuration with exceptionally high breeding efficiency is attained, and to lower the mass-to-charge ratio for heavy elements thereby alleviating the required acceleration gradient in theroom-temperature section of the LINAC.The results are discussed and extensively compared with predictions from the ebisim charge breeding simulation code. In order to explain discrepancies in the charge breeding efficiency and an apparent excessive electron current density for heavy elements, the effect of applying different electron-ion impact ionisation models was investigated, as well as contributions from collisional excitation followed by auto-ionisation.