I believe he could've been far more powerful than he was at the time of his death. Two things limited him: 1) I've heard the loss of his limbs reduced his overall force power - this is probably EU since it's not actually mentioned in the movies other than "he's more machine now.." 2) I believe the Emperor limited Anakin's growth in the powers of the force. Sidius wouldn't want his apprentice to kill him as he did his master, Plagueis (canon- mentioned in RoTS), so he would keep Vader's knowledge of the Force suppressed.
Hopefully, with the new TV show focused on the timeline between RoTS and ANH, we will see more of how powerful Vader really was after the whole limbs cut off thing. Other than force choking helpless peons around him and dueling with either an old man or a young, untrained kid, we didn't see Vader really flexing his Force knowledge in the OT.