I’ve shot in CA and never been to a match that was restricted to capacity by MD. Most shooters that weren’t grandfather’d either shoot in 10 round friendly divisions, but never seen or heard of other divisions being forced to 10 rounds.
The law does not restrict out of state competitors entering the state or possession, it only prohibits, export, import, or sale.
CA PC 30665, mags laws don’t apply to nonresidents.