Dino,
While most of us on this forum spend a lot of time with NASA, I and others also participate in PCA events.
1 Not much relevant activity from LA area PCA
2 Orange County PCA very active. Several 944 guys active. AX's at El Toro (closing soon or already) and now a local stadium or airport, not sure. They do a few big track events, but not a lot.
3 GPX region, or Grand Prix, does a lot local to you. Several 944 guys. They are also well known for an event at Streets of Willow called the "Day away from Work" Autocross that is more of a DE with an AX at the end.
4 San Diego Region is extremely active with a LOT of 944's at every level. Lots of AX in San Diego and 6-8 big track DE/ Time Trials and including a couple of PCA races each year.
In Addition, POC holds a lot of big track events when you are ready for that. Three or Four serious 944 racers active.
So, for AX, PCA is the most active. You can run with all of them (They play well together). For Big Track events, NASA and PCA have DE and Time Trial with about equal number of events and 944 players. You can do both with only a few conflicts. For racing, the most 944's play in NASA. PCA has fewer local races, but is still fun and you will have a few 944's to play with. POC has the most events, but fewer 944's participating. All run quality events at all levels.
Chuck