SAN FRANCISCO – Kia Motors America is elated with the success of its quirky Soul cross/utility vehicle, but unsure if it wants to stretch its luck with additional variants.

Sales of the Soul through June were up a whopping 156.3% from year-ago to 28,637, far outpacing competitors such as the Scion xB (9,852) and Nissan Cube (15,530).

Nevertheless, Kia officials aren’t certain derivatives of the boxy 4-door would make sense, fearing such models largely would appeal to those already interested in the current model.

“(The Soul) tends to be a niche vehicle,” Orth Hedrick, director-product planning, tells Ward’s at a Sportage media event here. “There is very high appeal to a very narrow group of buyers.”

Kia hinted at a future derivative with the convertible Soul’ster concept shown at the 2009 North American International Auto Show in Detroit.

But in order for Kia to green-light a production Soul’ster, Hedrick says “You have to have broader appeal” to justify the development time and money it would take to bring the model to market.

Because of its drop-top nature, Hedrick notes a production Soul’ster would be an “almost all-new vehicle,” due to structural and weight differences from the current Soul.

“The business case is really what we have to keep studying all the time,” Hedrick says, noting developing and selling Soul variants would be a challenge, “especially in this environment.”

When Kia launched the Soul in spring 2009, it had high hopes the vehicle would expand the reach of the Kia brand to more stylish, affluent markets. And, so far, that is happening, says Tom Loveless, vice president-sales.

Loveless says the 88% increase in Kia’s northeastern U.S. market share during the last 18 months is attributable largely to the Soul. The region includes such markets as New York City and Boston.

“We’re very fortunate the product is relevant in all parts of the country, but clearly the Northeast is a success for us,” he says.

While the Soul has been well-received, Kia already is thinking ahead to improvements. As part of its mid-cycle change expected in ’12, Hedrick says the Soul will get more up-level content, as well as new front and rear styling.

He also says Kia will remedy issues cited by owners of early production models, without going into details.

cschweinsberg@wardsauto.com