Hmm, It isn't entirely stock, it has franco kernel and a custom recovery. Cant I download the stock 4.3 image from google, wipe cache and dalvik cache then flash the 4.3 update without wiping data?
As I said you will probably need to flash the stock kernel and recovery from the
4.2.2 image first, then you can apply the 4.3 update when it is released. Flashing the stock kernel and recovery will only take a few seconds following the instructions.
The image from Google is not a flashable zip like you are thinking. The OTA update file itself may be sideloadable or flashable in a similar way, which probably wouldn't require any wiping.
The factory image however is a different beast altogether. Factory images need to be flashed in fastboot and require a full device wipe.. but we use them as a last resort when we've screwed something up on our Nexus devices. A factory image restore can always be used to flash your device to stock, out of the box condition.