MODAL is an optical design and analysis package targeting the millimetre and sub-millimetre region of the electromagnetic spectrum. It is being developed at NUI Maynooth with the aim of integrating advanced modelling techniques and access to High Performance Computing into a user-friendly and yet very powerful tool for an (quasi-)optical designer. MODAL has been recently extended to allow integrated simulation of custom corrugated horns and dielectric lenses. This made it possible to model an existing instrument (QUaD), with the goal of optimising its performance. Here we present new results from analysis of the predicted performance of the QUaD telescope, with particular emphasis on polarisation information. They were obtained by using MODAL to model the whole telescope, with the distortion of the primary accounted for, for a range of component tilts and separations.