Wiki here.
Basically, the Japanese Zero was superior to the American Wildcat, then we captured one intact. We test flew it and found its strengths and weaknesses. The result was the Hellcat with which we designed specifically to kill Zeros and it worked.
There have been several instances where there have been celebrity defections where the pilot has landed the latest ride intact.
What was the response of the receiving country? Are there any instances quite this dramatic?
There was the FW 190 which was superior to the Spitfire. The British got a hold of one and was able to modify the Spitfire
In April 1943, the two Jabo units were amalgamated into Schnellkampfgeschwader 10 (SKG 10) which switched to night operations over southern England, a role in which the Fw 190 proved unsuccessful, taking heavy casualties from the de Havilland Mosquito night fighters. On the night of 16/17 April, on this unit’s first operation, four Fw 190s which were attempting to attack London, got lost over Kent. Three of them tried to land at RAF West Malling: Yellow H of 7./SKG 10, flown by Feldwebel Otto Bechtold landed and was captured, his Fw 190 later being evaluated by the RAE at Farnborough; another Fw 190 of 5./SKG 10, flown by Leutnant Fritz Sezter landed several minutes later. When Setzer realised he had landed on an enemy airfield and attempted to take off, his aircraft was destroyed by an armoured car. Setzer surrendered to Wing Commander Peter Townsend. A third Fw 190 undershot the runway and was also destroyed, the pilot escaping with a concussion. The fourth Fw 190 crashed at Staplehurst, killing the pilot.
About the modifications .
The British Supermarine Spitfire was facing several challenges by mid-1942. The debut of the formidable Focke-Wulf Fw 190 in late 1941 had caused problems for RAF fighter squadrons flying the latest Spitfire Mk Vb.[2] By this time Rolls-Royce engineers were already working on a new version of the Merlin incorporating two-stage superchargers; the combination of the improved Merlins and the Spitfire Mk VC airframe in a “stop-gap” design allowed the RAF to combat the Fw 190 on equal terms. In a second stream of development Supermarine was working on an improved, reinforced Spitfire airframe which incorporated several new features and was designed specifically for the Merlin 60 and 70 series engines.[3] This new airframe later formed the basis for the Rolls-Royce Griffon powered Spitfires.
Victor Belenko flew his Soviet Mig-25 to Japan.
Among various others .
It should be noted that there are disagreements over how much intelligence was due solely to obtaining the Zero. The F6F was already in design based on reports from the field.