In single target patchwork fights haste is currently still ahead of mastery on PTR but there looks to be quiet a few cleave/aoe fights that will utilize Chain Lightning in SoO, which will suggest that mastery maybe be better just due to the encounter types. I'm personally going to stick with my current reforge and gem priority for the first week to can see what the encounters are like and what my jobs will be during these encounters. If I find that I'll be using a lot of CL then I will start testing out a bit more mastery. I also know Tickletotem is doing a lot of simming and currently writing a guide for 5.4 Elemental, so if you're still not sure just wait for that to arrive.
Hope this helped a bit