Still unsatisfied with the initial high speed result, we followed our instinct about what we thought was the problem and unplugged the front roof fan. Sure enough, with the two remaining fans on high speed, the CPU knocks 4°C from its delta T for a much healthier result of 50°C (although our GPU delta T did increase slightly). What this demonstrates is that the front roof fan was actually exhausting cool air from the case before it could reach our CPU cooler's fan.
Antec's own cooling setup evidently isn't optimal, as it's actually better to disable one of the 140mm fans that even more annoyingly can't be repositioned anywhere else in the case. Along with other minor niggles like the lack of anti-vibration rubber anywhere, relatively small space behind the motherboard tray and non-removable drive cage, this means that while the GX700 would still serve you well in most regards, it narrowly misses out on a recommendation