BARCELONA—In the sulfurous debate over the future of Spain's wealthy Catalonia region, Xavier Sala-i-Martin is the most notable and quotable member of a self-styled truth squad of pro-independence Catalan economists.

When the head of the American Chamber of Commerce in Spain warned in October that talk of a breakup was making foreign investors skittish, Mr. Sala-i-Martin retorted on Catalan radio that Spain was a "midget market" and globalized Catalonia would thrive on its own.

...