This sounds pretty speculative to me. The XBMC crew think there may well be a hardware limitation, e.g. insufficient video memory to drive a 1080p output.
I would think the relevant comparison isn't with getting devices to sync or play files they're not supposed to, it's with getting them to output at resolutions they're not supposed to. For example, the A5-based iPad 2 and iPhone 4S can output at 1080p using the HDMI adapter, whereas the A4-based iPad, iPhone 4 and iPod Touch can only output 720p using the same adapter. A5 devices have been available for 12 months and no A4 devices have yet been enabled for 1080p output over HDMI, which I would have thought would have happened if it had just been a matter of editing plists. So I would be surprised if the launch of the A5 ATV3 would make it any easier for the A4 ATV2 to output 1080p over HDMI.
However, I hope you're right.