It depends on your rifle. Some rifles will enjoy a significant improvement, others not so much. You'll only be able to tell by doing a side by side comparison with all other variables the same.
It makes a measurable difference in most loads I've tested. Other bonuses include fewer steps (less lubrication/cleanup) and less work hardening of your cases. I neck size only for all of my bolt actions. It is worth the price of the extra die IMO. Sorry I can't tell you "your groups will shrink by X amount". Only way to know it to do the comparison.