This is an old question but I simply must put the idea that using thin-set on walls is a good option. Thin set is a motar designed specifically for floor installation it's gritty doesn't stay on the wall well when applied. Now that isn't to say it won't work it will, provided you use a cement based backer board such as durarock or hardi-backer. But here is a key walls tend to move and swell more than floors do and thin-set being concrete and gritty isn't great at holding up to the stress. Even the modified/fortified variations have limits.

Mastic is designed for walls has outstanding bond and durability ease of application etc. And the only way water would be an issue is if it was soaked with it and remained so for weeks. In a kitchen that isn't a problem. I also use it for tile showers/tub splashes. Sheet rock isn't recommended for wet areas like showers and tub splashed but in a kitchen it's okay. Thin-set will not bond to sheet rock reliably. So if anyone decides to use it instead of mastic make sure you use a good cement based backer board.