I would recommend getting a copy of T/Cs manual for the NewEnglander & reading it carefully!
The MAXIMUM suggested load in my manual is a volumetric measure set at 100 grains for both shot and powder!
With the mag spark there is not much blow-back, and with that 150 grain (volume) shot charge you are probably exceeding maximum pressures by quite a bit! That could really "blow" a shot pattern.

As for the mag spark, I use one in my NewEnglander and see no impact on shot patterns, but I'm staying very close to recommended loads.