He had been given little incentive to learn to use it better. Surely SHIELD could have made a project of him and found a way to enhance his natural ability much like the Extremis did.The idea that SHIELD wouldn't have brought him in and either trained him to use his powers more effectively for their purposes (or just locked him up) when they first found him is pretty much completely unbelievable. No way Fury would have just let an asset like that go to waste. If they had made it less impressive, like little candle flames on his fingertips or something like that, I might have bought it.
It wasn't much more than that. It was a low-level power that he had limited ability to control.
Why was it necessary for May to inject him with the Extremis then? It was her double-dose that set him off.He burned someone to cinders in a few seconds even after the centipede formula wore off and as far as we've seen, he had only really tried to develop it beyond showing off to girls for that few hours he was in their lab.
I think you missed the shot where Chan/Scorch saw a tray of the hypodermics containing the Extremis formula, which were in the foreground of the shot and closely focused on by the camera for several seconds. It was after this that he burned his SHIELD handler to death. What that tells us is that he re-injected himself with the serum in order to give himself a new power boost. That's why he died a few minutes later -- because, as the bad guys said in dialogue, without his fireproof platelets, he'd have no protection against the serum's explosive properties.
He is a human who is both fireproof and able to create (and control) flame from his hands. Limited or not that isn't someone you let walk around unutilized in a world where Captain America, Hawkeye and Black Widow are considered superheroes.So the enhanced abilities you're talking about only existed because of the serum. His own unaugmented ability was far more limited.
Who says Extremis was even the reason he was able to perform at that level? How would they even know that his powers would be enhanced? It may just as likely have been a mental block from a lack of confidence and the discouraging warnings of SHIELD. This is the most powerful naturally occurring (not engineered or the result of an experiment gone wrong) meta-human ability we have seen in the MCU. I just don't buy it.