Revan never beat the emperor, he simply overcame his bonds to him. So in a way he did beat him mentally, just never in combat. Revan grew to understand the force as it was meant to be understood. The jedi were stubborn with peace, and the sith were blind with hate. Revan was so strong because he believed in Passion and peace, two principles that were divided between the sith and jedi for many years. It is hard to say for canon purposes, but Revan could probably wipe the floor with current generation jedi and sith alike, such as sidious/palpatine or someone like mace windu, due to his understand and strength in both sides of the force.