My understanding of using a small primer in a cartridge previously designed for large primer is to withstand more pressure, ie. a little hotter load than normal, to gain velocity. IIRC it started as a fad with the 308win in match ammo.
The theory as as I've heard is the case head web has more meat around the primer with small vs large, less square miles of primer surface for the pressure to push back on, and thus less prone to blow a primer out with a hot load.
Unless that velocity gain is your goal you probably won't see any improvement in anything that large primer cases will give. I've not paid any more attention to it than that, I've never tried it, and I have no idea about load data.
The increased pressure tolerance is a nice side benefit in the .308, but the original intent in that chambering was to provide a more consistent ignition and smaller velocity spreads in the ammunition used by the United States Palma Team in the Long Range World Championships held every 4 years. My understanding is Tom Whitaker lobbied Lapua to produce the first batch for the team and it became widely available thereafter.