We have eaten at Coral Reef many times and we always arrive well before our ADR time. Only once were we seated on time. We probably average 15 minutes past ADR time. Service is sometimes good and sometimes slow.
Even if you get seated on time and have good service, you are talking an hour for dinner plus the walk to World Showcase, putting you there at 8:30. It is feasible as long as you aren't expecting a great viewing location.