Only the Xiamo Mi3 and Note 3 have this now I think.
Anandtech - Xiaomi Announces the MI3 - Tegra 4 and MSM8974AB inside
http://www.anandtech.com/show/7276/xiaomi-...sm8974ab-inside

Excerpt from the article:

Xiaomi makes the first (to my knowledge) public disclosure of MSM8974AB, which is analogous to the changes we saw between APQ8064 and APQ8064AB.

From 8974 to 8974AB, Adreno 330 GPU clocks climb from 450 MHz to 550 MHz,

LPDDR3 memory interface maximum data rates go from 800 MHz to 933 MHz,

and the ISP clock domain (I think Xiaomi might mean the Hexagon DSP here) goes from 320 MHz to 465 MHz.

8974 comes in both a bin with the 4 Krait 400 CPUs clocked at 2.2 GHz (really 2.15 GHz) and 2.3 GHz (2.26 GHz) with slightly different pricing, while 8974AB comes with a Krait 400 clock available only at 2.3 GHz.

Process is still TSMC 28nm HPM, but I suspect that the AB variant might have the high k dielectric and/or transistor mix tuned slightly differently based on a few rumblings I've heard recently.