It may not be his Plan A, but as bailout negotiations come down to the wire, Alexis Tsipras is risking his country’s ejection from the euro. For some prominent supporters of his left-wing Syriza political movement, there could be no better outcome. In one fell swoop, they would be free from the yoke of their eurozone masters.

They should be careful what they wish for. An exit would almost certainly mean that the state of Greece’s economy, which has already shrunk by a quarter since the crisis began, would get worse—possibly much worse—before it gets better.

There is not much recent precedent for a country leaving a currency union while simultaneously defaulting on its foreign debts. Most of the nearest parallels relate to devaluations that accompanied debt crises over the past 20 years in Asia and Latin America.

These examples suggest that the value of a national Greek currency would plunge. When Argentina broke parity with the dollar in 2001, the currency sank toward 4 to the dollar, before coming back. After Mexico’s devaluation from a pegged exchange rate in 1994, the peso sank from about 3.4 to the dollar to almost 8 in the following year.

Ariel Burstein, an associate economics professor at the University of California, Los Angeles, who has studied large devaluations, says there is no way of predicting how far the drachma would fall: “If they let their currency float, who knows where it’s going to stop?” he asks. In such episodes, he says, exchange rates often overshoot way beyond the level that economic fundamentals would suggest, before settling back.

Traditionally, devaluations are viewed as providing a short-run spur to growth. They boost economic activity as people buy domestic goods instead of now more-expensive imports. The volume of exports, now cheaper in world markets, begins to increase—though that effect takes longer because exporters take time to find new customers. The U.K. and Italy both benefited from a growth fillip after deep devaluations following their ejection from the European exchange-rate mechanism in 1992.

In practice, a return to the drachma would likely be much more traumatic and any immediate boost swamped by economic disruptions. In Greece, the banks would be bust under the burden of a big currency mismatch. They would owe large sums in euros—not least to the European Central Bank—and most of their assets would be redenominated in devalued drachmas.

Even if they could enforce loan contracts to Greek companies in euros, that would shift the mismatch to their local borrowers, now earning drachmas instead of euros. Either way the banks would be in trouble, and a banking crisis accompanying a devaluation—as in Iceland in 2008—is invariably toxic for growth.

Devaluations are usually followed by shifts in two key metrics: the current account and the government budget balance. Greece’s current account—the measure of the balance of trade in goods and services combined with interest payments, which is now roughly in balance—would swing into surplus, largely because of a big drop in imports. Mr. Burstein says that large devaluations can be accompanied by falls of 50%, 60% or 70% in imports.

In addition, Greece’s fiscal picture would improve rapidly. Inflation would accelerate with devaluation, boosting tax revenues, while public spending—for example, on salaries and pensions—would usually be slower to respond.

There would be another likely effect that devaluation’s proponents in Syriza are less likely to trumpet: the effect on income distribution. Because of the rising price of imports, inflation increases. But in postdevaluation economies, it usually increases more in goods than in services.

Mr. Burstein says that because the poor tend to consume more of their income in goods than in services, they get hit harder by inflation than the better off. Richer people are also better able to protect their assets.

The hope of the devaluation proponents therefore has to lie in the longer term. After their traumatic first postdevaluation years, Mexico and Argentina both started growing, in Argentina’s case rapidly.

Mr. Burstein says economies do tend to grow more in the longer run after devaluations, but that is partly because they are playing catch-up on the ground they lost in the immediate aftermath. They are also often boosted by other factors such as debt defaults, which reduce transfers to foreign creditors.

After 2001, Argentina was helped by a soybean boom; Mexico’s growth in the late 1990s was aided by the newly created North American Free Trade Agreement. In general, he says, “It is very hard to pinpoint exactly if it is the exchange-rate devaluation or other factors that cause this future growth.”