I'm a 650B fan. That said, I have a 29er. If I lived in an area with more technical riding, I'd be all over a 650B. But for fast fire trail riding, which is mostly what SoCal MTB is all about, a 29er is the better weapon.
I definitely notice a real-world performance difference between 26 and 27.5. It's not as dramatic as the leap between 26 and 29, but it's definitely there. 650B absolutely smooths out a trail compared to 26, in my experience, and gives you a measure of "steamroller" action over small bumps and obstacles. Again, not like a 29er, but more so than a 26.
For what it's worth, I've never particularly cottoned to the idea that 29ers are only for people 6 feet and up. That said, I'm around 6 feet myself, so maybe I'm not the best judge. But I built up a 29er for my girlfriend, and she likes it a lot, no fit issues to speak of. With all the cockpit sizing solutions available these days (stems like the Syntace Flatforce and various negative rise bars) the math of 29er fit has really changed. I think the idea that they only fit taller people is kind of obsolete at this point...