but is there a gain or plus side to go to the magnum load? (150g)

I'd say generally NO! If things are erratic at 100 grains it will probably get worse at 150 grains.

If you are trying to find the MOST ACCURATE LOAD with any given bullet, you must use use loose powder so you can vary the charge in smaller increments than is possible with pellets. Sometimes a pellet load will turn in excellent accuracy, but often they won't!