You mention that the temperature was about 100 degrees F. In my testing, I've found that valve lock on these guns takes place at about 96 degrees F. Above this temperature, muzzle velocity falls dramatically - and with it the point of impact, of course.
At a guess, it may be that your QB78 was suffering from valve lock when shooting the JSBs. Running down two sets of Powerlets may have cooled the gun enough for it to have stabilised below 96 degrees - even if the ambient temperature remained higher - and the Premiers were thus shooting at full muzzle velocity and to the correct point of impact. At this temperature, your QB78 was probably shooting the Premiers at close to 800fps with the XP Tune Kit installed.
This could at least explain the elevation change in the point of impact.
Archer Airguns Inc.