He also had a good chance against 3rd form Frieza after Piccolo was getting owned. Also, I think his shots against Final form Frieza were at 100%, because 50% never stood a chance against SS Goku, and I don't recall Gohan vs FF Frieza pre-SS Goku. That scene is anime only anyways though.
Pre-SS2 though, I don't think Gohan's rages ever really did anything. Against Raditz, his mega headbutt didn't even knock him down. It just hurt him enough that he couldn't break Goku's full nelson. When he did his rage spike against Nappa after Piccolo died, Nappa deflects it with one hand. When he raged after Vegeta beat the tar out of Goku, he only got a few shots in, most of which were dodged or deflected. Recoome literally deflected his Masenko with his breath. After Krillin was impaled, he had a brief advantage of 2nd Form Frieza, but only caused a few bruises at most. He almost got 3rd form Frieza, but that just convinced him to go to his final form right then and there. After Frieza, it's never brought up again until Cell (unless you count the Garlic Jr. Saga).
---------- Post added 2013-05-21 at 06:00 AM ----------
Part of Gohan's story is that he gets much stronger when he's angry. This is something that is unique to him. I think the official reason is that his potential strength is stronger than Goku's or Vegeta's, but he can only unlock it while angry. Vegeta and Goku don't have the same hidden potential as Gohan, and get stronger through training and fighting.