Phantom Spey
One aspect where the Spey was outperformed by the J79 was the reheat nozzle design. The translating shroud on the Spey had a large base area behind the inner petals so even when the nozzle was fully open in max reheat there was still a lot of base drag. There was talk of a true convergent/divergent nozzle being designed for the Spey but as far as I know it never materialised.
I recently re-read Sir Stanley Hooker's autobiography Not Much of an Engineer. In one chapter he describes his experiences when Rolls Royce sold the Spey 202 design to the Chinese. In the mid 1970's we (HSA Ltd) hosted a visit to Holme on Spalding Moor by a combined Rolls Royce and Chinese delegation. We had been told they could see any aspect of the engine and it's installation in a UK Phantom but not view the radar or look in the cockpit. I expected lots of detailed questions about the clever bits of the installation such as the intake ramps and bell-mouth. In fact the thing that seemed to excite the Chinese the most were the little spring-loaded fingers that sealed the gap between the rear fuselage and the translating reheat shroud. I have occasionally wondered how they got on with the reheated Spey and whether they are still in service.
Walbut