I never used Slicing, however from what I see now: how you will share iPadHD resources with 3x for iPhone 6+? All that a bit complicated and integration with current Apple idea of 1x, 2x and 3x will requires a lot of work and testing.
I don't understand why they just don't allow developers specify which file will be used on iPad and iPhone or specific device.
So, for example I have my own texture format that encrypted in 2 sizes: "texture.pvr" for iPhone and "texture.pvr" - for iPad. Yes, they both have the same name, but located in a different folders, so cocos2d just loads them easily, depending on the device type.
And yes, there is almost such option "New Data Set":
But if I drag 2 textures with same name in that "Data Set" for appropriate device type - Xcode will place it inside folder "Data.dataset" and so rename it like that: "texture.pvr" and "texture-1.pvr". And now cocos2d should be fixed for that(probably again add -hd and -ipadhd, lol ), but it's sounds like impossible fix because of a lot of refactoring and new old code, if you know what I mean.
Ahh, all can be just so simple, if Apple give us an ability to indicate that for example folder: "resources-phonehd" will be used just for: iPhone 4,5,6 and "resources-tablethd" for iPhone6+, iPadHD.
Did I miss something? As I said, never used slicing. And from my point of view it's just designed for applications, but not games...