The conventional wisdom holds that we shouldn’t expect too many tech miracles from the iPhone 5. The latest iteration of Apple’s popular smartphone will mainly just be an incremental upgrade over previous versions. Ho-hum.

But what about an economic miracle? In a new research note, JP Morgan’s Michael Feroli estimates that iPhone 5 sales could boost U.S. economic growth in the fourth quarter by up to 0.5 percentage points. That could mean the difference between a disappointing economic expansion and a half-decent one.

Feroli’s calculations are fairly straightforward: He figures that the new iPhone 5 will sell for $600 per unit. If each phone contains about $200 in imported parts, then each iPhone sale adds about $400 to GDP. (Imports get subtracted from GDP calculations.) If Apple manages to sell 8 million iPhones, then that’s a $3.2 billion boost to the economy right there, increasing annualized fourth-quarter growth by 0.33 percentage points. If the government statisticians add in a hedonic adjustment to account for the fact that new iPhones are superior to the old ones, that could boost growth numbers even further.

Is that unreasonable? Could the iPhone really prop up the U.S. economy singlehandedly? “This estimate seems fairly large,” Feroli concedes, “and for that reason should be treated skeptically.” On the other hand, he estimates that sales of the iPhone 4s appeared to have boosted growth in the fourth quarter of last year by 0.1 to 0.2 percent. And this release is set to be even crazier.

If the iPhone 5 did boost fourth-quarter annualized GDP growth by 0.5 percentage points, that would be quite significant. Right now, most economic forecasters expect that the U.S. economy will expand at somewhere between a so-so 2 percent pace and an anemic 1.5 percent pace in the fourth quarter of 2012. But when you add in the iPhone factor, Feroli notes, that “would limit the downside risk to our Q4 GDP growth projection.”