David Brooks on Friday wrote about the merits of what he calls "rigorous theology." Moderate feel-good religion doesn't cut it, he says. He says that more rigorous religion is what motivates people to behave sexually, and that if Mormons refrain from coffee, that willpower must extend to the rest of their lives. Even setting aside the implicit "without god all is permitted" issue, his premise is dubious. The abstinence movement shows clear examples of religion not motivating people to behave sexually. Evangelical sociologist Mark Regnerus's book about evangelical teens describes how they compartmentalize their sexual behavior from the rest of their religiosity.
As for whether discipline in one area extends to others: I'm not aware of any research of the sort. My understanding of the psychology literature is that willpower is limited: if one is exerting willpower in one area, one has less will power for other efforts. On the other hand, a life-long dietary adherence requires no willpower at all. It's just habit, and habit requires no thought. In fact, people who break dietary restrictions for the first time describe the will-power that it takes to take that first habit-breaking step.
David Brooks keeps kosher himself, so I would think that he would have realized that habitual dietary laws don't impact will-power at all. Perhaps in the throes of Passover bread deprivation (his op-ed was published on the 4th day of Passover), he wanted to believe that his week without bread was bettering himself. Now that he can go back to bread, perhaps Brooks will realize how much he was romanticizing religious strictures. Or perhaps he'll continue keeping Passover for another month because the first week was so rigorous.