Aluminum engines also have better heat transfer, which can reduce engine hot spots and tendency to knock.
When the limit on power is engine knock, which is often the case, aluminum engines with the same specs/dimensions would be more knock resistant and tend to be capable of more power.
For example, with more knock resistance, one thing commonly done is increasing compression/boost/ignition advance to increase the power output of the engine.
If all the engine settings remain the same, such as when the engine tune is not adjusted for the change in materials, the cast iron would likely make a small amount more horsepower. This happens because the cast iron combustion chamber steals less heat during the combustion event, and more heat would be conserved for actual work.
A stronger cast iron engine block would be an advantage in some special circumstance where the engine is limited by part strength instead of knock, such as a low compression forced induction methanol engine with large amounts of boost and nitrous.