I've noticed that in using Unique and loading medium loads in .357 and .44 magnum that magnum primers open up the groups at 25 yards about 3/4 to 1". I consider 1.5" good enough but 2.5 is much too large. This is the same even changing the amount of powder and bullet.
In no way have I done a thorough test but have tried both magnum but don't bother trying the magnum primers. 98% of my shooting is with medium loads that is the only certainty I have experienced.