Dear OP:
I wish it were your ex-wife on here asking us for advice on this because I'd like to tell her that *I* did the same thing; Ten years ago I moved my young DSs (they were 9 and 6 at the time) from New Jersey to Massachusetts. Granted, that's not as much of a difference as moving from PA to the west coast, but it's not local as far as seeing family back home in NJ, on a regular basis.
I'd tell her that, in retrospect, MANY days I wish I'd never moved up here. I wish I would've had my DH move to NJ instead. I regret forcing my DSs to grow up with not seeing their dad on a regular basis. I regret taking away the opportunities for my DSs to grow up seeing MY family on a regular basis. On a more personal level, I regret not being there on a daily basis when my mother was dying (I was there for 2 weeks while she was dying though).
I'd tell her that NOBODY, not even an excellent step-father can take the place of a loving father, and to move her children from seeing their father on a regular basis is NOT in the best interest of the kids, no matter what. If the father was not involved in the kids' lives, then that'd be a different story, but you are very involved.
Even though she thinks they'll have a great life if they move...and they MAY have an excellent life out on the west coast, she'll NEVER be able to go back and let her kids grow up with their father nearby.
Anyway...10 years later (my DSs are now 19 and 16) I do regret moving my DSs to another state, despite the fact that we've had (and are still having) a wonderful life in here MA. There are more important things in life, and a child growing up with their parents nearby is one of them.
My best wishes to you, I hope it all works out.
P.S. For the record, my ex-DH did NOT give me a hard time at all about moving; there was NO battle. From the time I've been divorced I've gotten along extremely well with ex-DH and I've always brought the kids to NJ several times a year and they spend the entire summers in NJ, as well as almost every holiday. (I figure, I have them all of the rest of the year, so when the kids have off, I take them to NJ). Looking back, now I kind of wish I was told that I couldn't move. lol