The 2020 International Maritime Organisation regulations (IMO2020) reduced shipping SO₂ emissions by roughly 80%, decreasing the cooling effect of sulphate aerosols on marine clouds, leading to a positive radiative forcing. Recent Global Climate Model (GCM) studies agree on a positive Effective Radiative Forcing (ERF) of ~0.10 W m^-2 from IMO2020. However, these studies rely on parameterisations for sub-grid scale emission processes with assumptions on primary sulphate fraction, particle size, and injection altitude, which contradict observational evidence for shipping exhaust plumes. Using the UKESM1.1 climate model, we conduct sensitivity experiments to quantify the impact of these uncertainties. We find that reallocating primary sulphate from the accumulation and coarse modes to the Aitken mode increases the IMO2020 ERF from 0.10 W m^-2 to between 0.19 and 0.31 W m^-2, and additionally increasing primary sulphate fraction increases this further up to 0.41 W m^-2. This sensitivity is driven primarily by the cloud radiative effect (ΔCRE) responding to an order-of-magnitude increase in modelled aerosol number emissions for the same sulphur mass, and is consistent with earlier shipping studies using other GCMs. Because recent GCM estimates rely on the same biased sub-grid emission assumptions, we argue this underestimate is structural across recent studies, and we find that the default-parameter experiment with a 0.10Wm-2 forcing significantly underestimates regional ΔCRE values relative to published satellite observations. An IMO2020 ERF 2 to 4 times the current consensus would explain a larger portion of Earth's energy imbalance since 2020 and of the recent global temperature surge.