Why Martlet deliveries did not weaken the US Navy

Would the war effort have benefited if all Wildcat and Martlet production had gone to the US Navy?

No; the United States lacked the carrier capacity to use all of them at that stage, while Royal Navy deliveries put the fighters to productive use.

The United States did not yet have enough carrier decks to use every additional Wildcat that industry could produce. The relatively small Royal Navy allocation therefore did not deprive the US Navy of fighters it could immediately employ. Later American carrier expansion coincided with Hellcat production, while Royal Navy experience helped make the Corsair usable at sea. Greater attention to American fighter pilots' operational proposals from the 1930s and to experience in the Mediterranean is suggested as a more useful improvement than changing the allocation.