There is another important variable besides chamber pressure and nozzle design to consider--how the engine pumps the propellants into the chamber. The J-2 engine, like the lower stage (and much more powerful but lower ISP kerosene burning F-1 engine) used a gas generator (basically burning oxygen and fuel, here hydrogen) to drive a turbine that pumped the oxygen and hydrogen into itself and the main chamber. The F-1 (using kerosene instead of hydrogen) was designed with high enough gas generator pressure to drive the pumps at launch, but the J-2, designed for vacuum operations, used a lower gas generator pressure to drive its pumps. The very low sea level ISP reflects not just the usual "back up" of rockets by sea level air pressure but also a turbo pump rendered anemic by the same back pressure.
Just today I downloaded software called RPA that simulates rocket chambers and nozzle performance for a wide variety of propellant types and chamber pressures and other conditions. My attempt to simulate the J-2A engine (an upgrade of the J-2) suggests, if I did it correctly, that a reasonable sea level ISP for the J series engines with the same type of chamber and nozzle would be a bit under 300 seconds, a reduction of 31 percent. In terms of thrust in Klbs, that would suggest a bit under 160, so at just under 110 poor pump performance at sea level apparently dings it another 22 percent.