"With the Magnificent 7, the market's up over 20%. That's the S&P 500 on a year-to-date basis through November," Mullaney notes. "If you strip them out, it's about half that, roughy over 10%."

While money managers "still feel comfortable owning these stocks", says Mullaney, based on their companies' strong earnings, there are possible warning signs ahead.

Among them, the potential for high volatility, what Mullaney calls "a high beta play."

"The beta of the Magnificent 7 is 1.47, which means they have about 50% more potential volatility than the overall market, either in an up market or a down market," Mullaney says. "And so it's something to watch going forward. The key thing is whether they can continue to produce the types of returns and the types of results that they have on a year-to-date basis, and really going back almost now two years."