It's been a while since I've read up on the JSF competition, but I wonder what the original performance requirements were (across all services!)? This thing didn't get designed/built overnight, along the way it had to through many design reviews (SPOviets)...chaired by fliers. Not defending the decision makers but the potential costs savings enabled these compromises. The thinking was the sensor networks, EW suites, C2 of drones, system of systems (remember that?) was going to make up for the shortfall in performance. The Vietnam lessons were mostly forgotten as one LM engineer was openly touting to me the benefits of removing the internal gun... I vaguely recall the original intent was to buy such a great number of the JSFs to replace the Eagles/Vipers/Hornets/Harriers, that the USG will get a great deal out of this. Once again, the taxpayers got fooled by slick PowerPoint slides by the MBA types.
Ever since 4th gen, the challenges (and costs) have always been about software/coding. Aerodynamics, structures, materials, mechanical systems, engines designs process are all very matured (incremental improvements only) thanks to all the test data since the Wright Brothers; in general, aircraft performance is based on a series of cost, weight, performance tradeoff analysis...the real variable is what value the computer can add to the aircraft.
Nerd rambling off...