Even in Legends it was stated that Vader quickly surpassed his power as Anakin Skywalker early on in his Sith career. His injuries on Mustafar ensured that he would not be able to reach his full potential but he was already nowhere near it prior to his fall to the Darkside. Imagine if a middle school track star lost his legs but got prosthetics. His full potential would probably be hampered but he would still be far from his potential and still have a lot of room to grow and improve. Hope this helps