Filter with Return: Cure for Vaporlock?

You are right, you need cold air. Two things going on.
[1] You are losing HP because the hot air the engine is breathing is less dense & makes less hp. Getting the engine heat soaked doesn't help either.
[2] The carb jetting was unlikely to be optimised for such high ambient temps. Meaning the higher underhood temps will make the carb richer from what the optimum was & this reduces power slightly.