The issue was brought up on the WoW-US forums, prompting me to go back and test.
Nothing has changed. There is still a Dispel first, then Damage. In other words, if you Dispel a shield, you will deal full Shield Slam damage, it won't be absorbed first. Or, if you Shield Slam someone at full health with a health buff, you will knock off that health buff and then do damage in addition to it.
Here's a very quick screenshot:
