Shadow priest/frost mage is an easily viable team. I've ran it 2k+ multiple times on my mage without tazik/synapse. As long as your priest is quick on heals, the mage polys, and the preist fears/silences at the right time its an easy game.
Both classes have a fair amount of peels, if your priest is getting focused either root or poly. If your mage is getting focused have the priest either fearbomb, silence, or terrify.
Oh, dont forget iceblock and dispersion.
Tl;Dr; It's an easily viable comp if played correctly.