I’ve dug up some more info. This looks like it could be an intended change but I don’t see it in the patch notes. Go to UnrealProjects/Intermediate/Config/CoalescedSourceConfigs and open the Scalability file. Towards the bottom you will find r.EmitterSpawnRateScale. These are set to 0.125, 0.25, 0.5, and 1.0 for EffectsQuality levels 0-3 in 4.8.0, but in 4.7.6 the r.EmitterSpawnRateScale is set to 1.0 for all effects quality levels.
Unfortunately when I try to change and save these values in notepad they don’t stick.
However, issuing the following console command adequately resolves the issue:
r.EmitterSpawnRateScale 1
Alternatively “apply global spawn rate scale” can be turned off on each particle emitter in cascade.