With 540 ilvl I just ran the standard hit>haste>exp>mastery>stam with 1 stamina trinket on some fights for the first 11 heroic bosses. For Siegecrafter and Paragons I opted for hit>haste>mastery>expertise>stam with more emphasis on stamina on Paragons. I was using the same for Garrosh until I realised what a weak boss Garrosh is compared to Paragons so it made me revert back into hit>haste>exp for the dps.
The mastery build is definately viable and useful. In fact it is the best survival build for several bosses. The problem is just that it is not needed unless you are in like 520 ilvl for heroic content, the bosses hit too weak for our current gear.
Also, I disagree that mastery is the most useful with Holy Avenger, that is more likely to leave you dead as you will have too much downtime when your HA is down. HA in general has been very lackluster in SoO to be honest. DP is in my opinion much better choice for a mastery build, and in SoO in general. Only real use I found for HA was bursting down Siegecrafter adds. It is pretty solid choice for Thok aswell but I opted for DP to be able to EF the raid more as tank survivability was not an issue.