The largest private-sector effort to tame medical spending in Massachusetts appears to be getting results, as doctors who agreed to work on a budget have cut costs by using less-expensive imaging and lab companies, and expanding office hours to reduce emergency room use. Health spending for patients treated through Blue Cross Blue Shield of Massachusetts’ pioneering “global payment” program grew 3.3 percent less in 2010 than for patients whose physicians were paid the traditional way, according to research published Wednesday.