I think he left a lot in the gym. There are stories about him going to war with heavyweights in the gym; Bojado had a chip on his shoulder about being the baddest guy in his gym and he would fight to prove it.
Steve McCrory. Won a gold medal and was said by Steward to be the most talented boxer out of the Kronk Gym and yet never won a world championship.
Show ponies aren't just prospects that flame out but guys who are more show than go, usually muscle-bound, go-for-broke, dudes with 6 packs, but whose power doesn't quite align with their physique and who often lack a gas tank or mental toughness. They can occur at the higher ranks. For instance, Anthony Joshua is a high performing show pony. Most are lower performing.
I still have a hard time applying the show pony tag to Lacy and especially Joshua. Joshua won the title from Klitschko who was the second longest reigning heavyweight champion ever. I will stop well short of calling either Lacy or Joshua great, but they both were very good world championship prize fighters that had deep amateur backgrounds. Legit dudes for sure.
Anthony Joshua was an underwear model masquerading as a fighter. A fat, short, stumpy part time boxer with quick hands and one combo in his arsenal beat him purely with the dog inside him. That makes Joshua the definition of a show pony, albeit a high performing one.
Well I guess that underwear model that won an Olympic gold medal and beat Klitschko in a great fight is a show pony according to you and a few others. Fighters lose fights. I don't look down on Louis or Lewis for losing to Schmelling and McCall either. Like Joshua did, they dominated their rematches.