After a long period of underperformance, small-cap U.S. stocks may be staging a comeback. While it is too early to say for sure whether this is the start of a sustained rally or just an occasional rise, recent data show encouraging signs for small-cap investors.iShares Russell 2000 ETF (IWM) gained about 28% over the past six months versus 23% gains in SPDR S&P 500 ETF Trust (SPY) . Earlier this month, the Russell 2000 topped 2,500 for the first time ever, as quoted on CNBC.Much of the early-year weakness i ...