In this work, we show that the UC quantum yield of visible-to-UV TTA-UC systems may approach the often-encountered spin-statistical limit of 20%. We do so by pairing six different annihilators with the purely organic, high triplet energy sensitizer 4CzBN which exhibits efficient ISC and a long triplet lifetime. The results show that the TTA-UC pair 4CzBN/TIPS-Naph achieve a record-setting 16.8% upconversion quantum yield (out of a 50% maximum), and high quantum yields are reached when using PPO (14.0%), PPF (13.0%) or TP (12.6%) as annihilators as well. We also show that the same set of time-resolved emission measurements can be used to determine the annihilator triplet lifetime, the rate constant of triplet-triplet annihilation, and the threshold excitation intensity, all of which are important parameters to probe when evaluating TTA-UC systems. The importance of having long-lived annihilator triplets is reinforced, as our results show that both the TTA-UC quantum yield and the threshold excitation intensity benefits from this. Using 4CzBN as the sensitizer limits the achievable anti-Stokes shifts, and our results are discussed in the context of extending the excitation wavelength further into the visible region. The development of high-efficiency vis-to-UV TTA-UC systems will require both new sensitizer and annihilator compounds, and finding avenues to control and alter the singlet and triplet energy levels of these will be crucial in order to combine high efficiencies with e.g., excitation with green light.