Nothing about a mirrorless body changes the EF lenses. The adapter places the EF lens to the proper flange distance and the characteristics of the mirrorless AF may focus the EF lenses slightly more precisely, depending on your legacy DSLR body that is the basis of comparison. Although the increased pixel resolution may highlight flaws in the EF lens, the increased pixel resolution is more likely to enhance the EF lens results. The vII/III models are more likely to benefit from the higher pixel resolution sensor, being designed specifically for Canon's newest sensors.