The Spey did have very good thrust at low level but the reheat light-up time was quite slow (about 5 seconds from memory). I flew one trial in 'C' fit when we had tests at 750 KIAS (the limit)and 250 ft (over the sea) and thrust was not a problem. I only flew a few sorties with the J79 and didn't have the chance to go high speed low level so cannot comment on the difference.
At Boscombe we had the pre-series F-4K (XT597) which we often flew clean (plus AIM7 on station 6 for c.g. reasons) and it was quite light so the opportunity was taken to try to get a Spey-engined F-4 to 2.0M. The profile we used was to climb to 48 000 ft at 0.95M then enter a shallow dive to accelerate until you reached 2.0M. The first time I tried it the rate of climb seemed quite low above 40 000 ft so I started the dive before early (cannot remember precisely how high) and I only reached 1.95M. The next time I persevered to 48 000 ft and reached 2.0M quite easily. We had a couple of very experienced F4 guys on the squadron and they had never been to 2.0M in a Spey-engined aircraft until they flew 597. Again, I never had the opportunity to fly this profile in a J79 mark so cannot compare although the high altitude flying that I did in one I recall as being much better performance qualitatively. The only other interesting performance point in a 'clean' 597 was 600 KIAS at low level, pull into a vertical climb and zoom until about 250 KIAS then pull over at 5 - 10 AOA until inverted and roll out; used to make about 31 000 ft. Happy days!