I don't think the PEDs hurt him as much as his moving up to a higher weight class hurt him. Even if he moved up naturally, it's unlikely there would have been any different. He would either have to gain the weight in muscle or fat and would lose something: speed and/or power. He's only been shown to have used PEDs during the De La Hoya fight not the Wright or Mayorga fights.
I think I said it earlier but I'll chime in one more time on Margarito and his gloves.
I don't see anything to say he had padded gloves against Paul Williams. Williams took some good shots late in that fight and still won the 12th rd like the previous 4, which Margs won, never happened. He wasn't marked or bruised up like Cotto was after his fight with Margarito. I suspect, but have no knowledge, that he did cheat Cotto and that may have been the first time though I'm leery of the second Cintron fight. I think he was Cintron's head and would have won anyway but he smashed that dude, who was looking sharp going in.
I will say Shane has had a couple of things working against him.
1) Weight: the more you move up the greater strain you put on your natural speed and agility.
2) His opponents: Oscar is a smart fighter with a great technique. If you made the mistake of thinking Shane wasn't going a tougher the second go round with Oscar your tripping. I don't care if Forrest's reach, height and the way he fights tall, would have given Shane problems at 147. Winky is just hard for anyone to beat.
3) Shane's fight style: The power punching looks cool and can fuck a lot of guys up, but the problem for Shane became that he was moving up and he wasn't hurting cats like he used too. Shane should have adapted to a more technical style but he had too much faith in that shit.
4) Age: Cats forget that Shane is old and the majority of these fights were in his thirties. With age you lose speed, strength, conditioning and agility. There is no amount of training that will stop this shit and after you hit 30 you are on the decline especially if you are fighting tough fights.
About the Margarito v. Williams thing I'm mainly focusing on the 11th round. Margarito landed some shots and Williams was making the same faces that I saw him make when he was fighting Martinez. He had surprise/confused look on his face like where the hell did this shit come from. Early in the fight William was eating the punches like lunchables. In the 12th round, Williams just threw an ass load of punches to keep the nigga off him.