In fact, "a sizeable number of clients" believe the Fed won't hike rates at all in 2015, leaving the funds rate anchored near zero percent where it has been for the past six years.
Read MoreFed now expected to stay lower for a lot longer
Kostin said the difference between the two views is pretty big: He forecasts that Fed tightening will cause the S&P 500 to end 2015 at 2,100, which would be little changed from its current level. If the client view should take hold and the Fed not raise rates, the stock index would escalate to 2,300, a gain of about 10 percent, he said.
"Adherents of the 'secular stagnation' thesis believe the financial crisis left the U.S. economy irreversibly weaker, and the economy will not be capable of tolerating higher rates," Kostin said. "Clients validate these views by pointing to forward market prices, which imply fed funds at 0.5 percent and 10-year rates at 2.5 percent by year-(end) 2015, compared with our forecasts of 0.6 percent and 3.0 percent, respectively."
"Secular stagnation," a theory advanced by former White House economist Larry Summers and others, refers to a belief that the U.S. economy is mired in a long-term period of slow growth that will necessitate low interest rates.
In the Goldman baseline forecast—which remains the primary call despite the upside risks Kostin expressed in the report—the market will see rising earnings leading to gains in the first part of the year, followed by a decline as the Fed hikes rates.
Read MoreFed closes year with 'optimistic' view on economy
"If interest rates in 2015 remain below our forecast, then equity returns may exceed our expectations," he said.
In that case, Goldman recommends clients stay with companies that focus on dividends and share buybacks.
A portfolio of 50 stocks that focus on those companies outperformed the S&P 500 with a 16.7 percent return, compared to the index's 14.4 percent (including dividends). Leading companies in that basket include ADT, Corning, Viacom, Hess and FedEx.
Read MoreStock buybacks expected to jump 18% in 2015