The neutron spin echo spectrometer IN15 at the Institut Laue- Langevin, Grenoble, has been constructed with a focusing mirror option, designed to increase the intensity for measurements at longer neutron wavelengths, and thus to reduce the minimum Q value by an order of magnitude to approximately equals 10-3 Angstrom-1, and to increase the maximum spin echo Fourier time by at least a factor of 2. In this paper we describe the results of analytical calculations and ray-tracing simulations which compare the performance of IN15 in its standard and focusing mirror instrument configurations, taking into account the mirror size and geometry, the wavelength, the wavelength distribution, and the incident beam intensity and divergence. We analyze the intensity profiles for both configurations, presenting the results as plots of intensity at the sample against minimum achievable Q in horizontal and vertical directions at the detector. We show that the gains predicted by ray-tracing simulations, for realistic instrument set-ups, differ considerably from those anticipated by analytical calculations. The deviations are principally attributed to gravitational effects, which rapidly worsen with increasing wavelength.