"I feel like he's going to get better and better the more he bowls and the more we play," Smith said of Steyn, who recovered from a fractured rib to take part in the three-Test series.
JP Duminy was named man of the match of the second Test, which the Proteas wrapped up in the final over of day four.
But on Sunday it was Steyn who ripped the visitors' hearts out - figuratively, although his possessed mannerisms after uprooting Brad Haddin's middle stump suggested anything was possible.
Steyn claimed the scalps of Michael Clarke, Steve Smith and Haddin in one fiery spell that spearheaded a dramatic collapse from Australia, who lost 10-90 as they slumped to a 231-run loss.
He finished with figures of 4-55 from 20 overs and looked just as unplayable as Mitchell Johnson did in Australia's crushing first Test win in Centurion.
Smith expected more improvement from world No.ranked Steyn, who lost his voice amid four demonic wicket celebrations.
"He carried an injury for a while before the series and didn't have a chance to bowl a lot going into the Test series," he said.
"I've always known he's probably a spell away from hitting his best and that the more he's bowled, the better he's going to get.
"Hopefully that spell he produced with the reverse-swinging ball really sparks him for the next Test match."
Clarke wanted his teammates to learn from the reverse-swing masterclass.
"South Africa showed us how to get the ball reversing, we didn't get one to reverse in both innings so we can learn from that," he said.
"I know how good he is and I know how fast he was bowling, so it was about trying to find a way to protect your wicket but unfortunately we couldn't do that."
Steyn was particularly fired-up after Smith opted against reviewing a confident caught-behind appeal when centurion Chris Rogers was on 69, with replays showing a healthy edge.
"His anger goes from very angry to extremely angry," Smith laughed.