Bottom Gun, who posted above, has it right. Therre is no difference in group size or performance in general.
What small rifle primers do in high intensity cartridges is to give more brisance, and thus more powder is being burned.
Place a large sheet of white paper about 10 fet ahead of the muzzle and fire a shot with a standard pistol primer. replace the paper and fire a shot using a small rifle primer. The latter will show less unburned powder granules, as demonstrated by less burns and scorch marks around the bulet hole.
The "larger groups" noted by the NRA were due to a psychological
reaction. Tests from a machine rest proved, in every caliber tested using small rifle primers, that the only difference was that more powder was burned. It is altogether possible that a given load in a standard loading book shows more powder than necessary. If it doesn't burn in the chamber and barrel, it is wasted. Adding more powder than necessary raises chamber pressure, not from the effect of more powder burned, but from the unburned powder taking up space in the case and the lesser available expansion space creating more pressure. Tests show that after the optimum amount of powder is introduced, restricting combustion space with a commensurate amount of Cream of Wheat, rather than additional powder, will raise presure identically. The only conclusion to be drawn is that you cannot have too much brisance, and many scientific tests by Frankford Arsenal (such as the addition of frontal ignition tubes) bear this out.
You might want to remember that unburned powder granules have a "sandblast effect" on a barrel, throat, topstrap and forcing cone. That is the downside of using a slow burning powder. Obviously, the effect causes deterioration at a very slow rate in some cases.