I remember those T-Bird sorties very well. It was my privilege to show back-seaters the fastest speed they were ever likely to travel on earth unless they went on to fly the Tornado. At 250ft over the sea, M:0.99 was around 660kts, depending on the temperature. The only reason to stay subsonic was fear of the drop tanks hunting and coming apart. You could see in your peripheral vision the noses of the tanks weaving around at high Mach. A few years later that fear was dispelled when we did the supersonic dive release trials of 1000lb bombs and CBU 87s with the centreline tank unfazed by the excursions outside its design envelope.