Is Safox X really more accurate with fast glass?

KL Matt wrote:

Hi all,

I have a K-5 and the FA limiteds. Sometimes focusing with my FA 31 @ f/1.8 isn't quite the experience I'd like it to be. That, and my DA 21 front-focuses in indoor lighting quite badly. So I'm carefully considering upgrading from my (recently purchased! Ouch!) K-5 to a K5II or IIS. But I don't think the low light issue would be quite enough to get me to take that loss - I'd need to know the new autofocus system is considerably better (faster, more accurate) with fast glass.

Does anyone have any experience comparing the K5 with the K5 II(s) using fast lenses, like the FA limiteds?