I noticed the iPad's 12w charger on the Watch accessory page today and was wondering the same thing. Seemed odd that they'd include it there otherwise.
My iPhone always has charged faster with the iPad charger so I would assume there might be a bit of a boost. It all depends on the battery. We need an iFixit tear down. Lol
The 12W charger charges devices faster because of the increased amperage output relative to the 5W (USB).
This is only the case on devices which specifically support it, as per Apple.
IMO, if the devices are not specifically named by Apple to support the 12W charger, then the increased output would most definitely affect the battery of the device perhaps negatively.
I most definitely hope Apple says it is supported!