Needing to match exit pressure to ambient pressure is also why increasing the chamber pressure gets you a higher efficiency in atmosphere, because it lets you use a greater expansion ratio for the same exit pressure. For example, the Rocketdyne F-1 and the SpaceX Merlin are both kerosene-oxygen rocket engines that use a gas generator cycle, and both have relatively similar specific impulse in a vacuum (F-1 304 s, Merlin 311 s). But where the F-1 has 7 MPa of chamber pressure, the Merlin has 9.7 MPa, which translates into a significantly higher specific impulse at sea level (F-1 263 s, Merlin 282 s).