Mobile US Inc. has almost singlehandedly lifted the 2015 stock performance for U.S. wireless carriers, which as a group exceeded the benchmark Standard & Poor's 500 Index after price battles cooled off. With Sprint Corp. down about 8 percent, Verizon Communications Inc. hovering in the unchanged region and AT&T Inc. up about 3 percent, it's T-Mobile's 47 percent gain this year that has catapulted the group's collective average way over the broader market.