METIS (Mid-infrared E-ELT Imager and Spectrometer) is the mid-infrared instrument proposed for the European Extremely Large Telescope (E-ELT). METIS will be the first instrument in the mid-IR that will actually require an Adaptive Optics system in order to reach a performance close to the diffraction limit. Extending Adaptive Optics for the mid-IR from the current generation of telescopes to 30-42 meter telescopes is technically challenging, but appears at first sight significantly easier than at visible and near infrared wavelengths. Adaptive Optics has been demonstrated to deliver Strehl Ratios exceeding 95% on 6-8 meter class telescopes at 10 microns, but achieving this performance on E-ELTs under normal observation conditions, requires that several higher order effects are taken into account. The performance of a mid-IR AO system drops significantly if refractivity effects and atmospheric composition variations are not compensated. Reaching Strehl Ratios of over 90% in the L, M and N band will require special considerations and will impact the system design and control scheme of AO systems for mid-IR on ELTs. The METIS instrument has finalized its preliminary design phase and in this paper we present the results of our performance estimates of the METIS AO system. We have included the effects of refractivity and composition fluctuations on the performance of the AO system and we have investigated how these effects impact the science cases for mid-IR instrumentation on an ELT.