It will be perceptibly faster when iOS 8 comes out. The new OS always kill performance on the "older" devices and encourages trade ups... A scam in my opinion.
I don't think the iPad Air and iPad mini will get the A8X, although I think that any iPad "Pro" is at least somewhat likely to get an A8X if one or more of these statements ends up true:
The iPad Pro has a higher resolution than the iPad Air. I'd say this is almost certain given the resolution and PPI of the iPad Air. Then better graphics in an A8X can help.
The iPad Pro is released in the spring or some time offset from the normal iPad releases. Giving Spring 2015 as an example, it may be that the A9 won't be ready yet, the A8 is already mid-cycle, and the 1st gen iPad Pro might have to coexist (not as a price-dropped model) with A9-based iPads.
Going off point [1], if the iPad Air has a pixel-tripled display then I can see it having A8X, but I think they are going thinner and not a lot more powerful this year, and maybe saving performance and resolution improvements for a later year.