Why Britain modified American-built escort carriers

Were British modifications to lend-lease carriers worth delaying their entry into action?

Yes; the changes reduced fire and explosion risks, and later American production adopted the lessons.

Britain altered aviation-fuel management, arresting gear, and related systems because two years of war had exposed hazards the original designs did not yet address. American builders then incorporated many of those improvements into later batches rather than treating the first British delay as wasted time. One concrete difference was that British practice drained aviation-fuel lines and filled them with gas, reducing the quantity of volatile fuel left near the hangar. The delay therefore bought fire and explosion resistance while also feeding combat-tested improvements back into continuing American production.