I live in CA, and read everything on the excellent CalGuns site regarding magazines (and other B.S. gun restrictions we suffer under). To answer the OP's question, you cannot bring magazines into the state of CA that have a capacity greater than 10 rounds.
That would be "import" of illegal magazines, which is a felony here.
There are some people who have and use hi-cap mags. Some are LEO, which are allowed an exemption. Some have hi-cap mags which they owned in the state of CA before the year 2000, which are grandfathered in. Most others are flat-out risking felony prosecution.
I've spoken with some of them (they are at every competition), and they invariably point to the fact that in the California penal code, there is nothing regarding the possession of hi-cap mags. IOW, you cannot make, buy, sell, import, give, etc, etc, but if you happen to just magically have them, it's not against anything in the penal code.
Still, this is California -- a very anti-gun state. There are many DA's who will be happy to launch a political career by prosecuting some out-of-state guy who is importing "lethal, illegal" magazines with "enhanced capacity for killing". You can't vote in this state, nor, presumably, can your family or friends, so this overly-zealous DA won't lose any votes by destroying your life, and he will gain the votes of the (majority) anti-gun crowd. Will he be able to prove in a court of law that you imported those magazines? Remember, if the morons in this state can find OJ innocent, they can find you guilty. Even if you win, you'll have been sitting in jail awaiting trial - will your job wait for you? Will your lawyer bills be paid for?
Lotta things to consider, hope I shed some light on some issues for you to evaluate.