if that, if you're lucky. . . You might gain 1-3hp with an axleback . . .
if you are considering an axle back, do it for sound; "I'm dubious" about the manufacturer's claims of performance improvements ("Hello Mr. Dubious!")
Apparently on highly modified cars -- like with superchargers, long-tube headers, high-flowing pipes, etc. -- the restriction of the muffler can become a factor, but like they said, the stock mufflers flow pretty well so it's not significant for a stock or lightly modified car