Increasing the pressure recovery in diffusers is an ongoing endeavour in engineering. In this paper, we analyse, based on extensive numerical simulations and experiments conducted for a wide range of half-opening angles, the effect of boundary layer stability on pressure recovery in diffusers under the influence of an unsteady rotor outflow. We present, based on integral stage design parameters, analytically devised and empirically confirmed correlations for the quantification of additional pressure recovery in diffusers under non-uniform, unsteady inflow conditions and their sensitivity to the half-opening angle. We demonstrate that the sensitivity of the pressure recovery to the half-opening angle is caused mainly by a change of the effective area ratio due to blockage effects and provide a simple ansatz to estimate the magnitude of this effect. The estimate is shown to agree with experimental and numerical data. We define a parameter, based on the diffusion of vorticity, to quantify the processes that stabilise the boundary layer and, for the first time, present a correlation that unites a wide range of different half-opening angles.