I have taken my 2011 Zephyr out in 18knots, so you can probably manage on a 10m or even less for your next size down.

I have been using the smaller bar from my 8m Catalyst and 11m C4 (2011) with the Zephyr. Steering seems fine, especially when using normal length lines. Controlling such a big kite in higher winds is actually easier than I thought it would be. It's just so well behaved, handles gusts really nicely.
I was mainly able to control the speed just by edging a little harder, and I wasn't even on full depower. Not sure how the 2013 C4 bar would go with the larger kite, as I think you would want that extra bit of throw just in case.

As for turning the Zephyr, I haven't really felt the need to turn it very fast.
You don't need to really do much with it, you just have to adapt to it's style with your jumping and transitions. It just sits in the sky and doesn't move, and you can do what ever you want underneath it, wipeout, and it will still be sitting where you left it.

I have the 2011, and by all accounts the newer versions are much better so keep that in mind.

I only really like taking it out in about 12+ knots at the moment, but that might just be because I have not perfected my light wind riding technique.
At 12 knots I will be comfortably going upwind, doing some jumps and back rolls, and it's heaps of fun. I can definitely get the kite up in the air from about 8 knots, but it's not a whole lot of fun, and never guaranteed to stay upwind.