Marx definitely did not believe capitalism would produce more surplus than scientific socialism. However, I doubt he believed it was a switch you could throw and suddenly have a more productive economy. His point was that you could approach the problem scientifically. Through hypothesis testing and experimentation you could develop something more rational than the anarchy of the market.

That said, communist governments haven’t ever found themselves in a position where they can safely experiment with a planned economy. The USSR was struggling for decades to defend itself from foreign adversaries. That’s why it developed a tightly controlled and powerful bureaucracy which eventually led to it’s downfall. It’s also why China and Vietnam decided it was safer to experiment with market reforms given that they are at an economic disadvantage to capitalist countries. Other attempts at building a socialist economy have been thwarted by either US backed coups or sanctions, as was the case with Chile and Cuba.