Sleep Talk is certainly viable on strong Choice users who get all the coverage they need from a few moves. Specs Yanmega is known for running Sleep Talk often so that it provide team support by absorbing sleep. I could see the set as:That would make it a fairly situational move. In order for sleep talk to be useful you'd obviously have to go for rest talk. If someone could give a good reason for why Restalk is useful for Therain forme, I'll gladly mention Sleep Talk and maybe put a set up.
Tornadus-T @ Choice Specs
Trait: Regenerator
EVs: 4 Def / 252 SAtk / 252 Spd
Timid Nature (+Spd, -Atk)
-Hurricane
-Focus Blast
-U-Turn
-Sleep Talk