Subterfuge is the best lvl15 talent for Sublety - we're talking about raiding anyway.
For leveling it's better to use Shadow Focus.
Is it really better for raiding? Until they change how it works with vanish, you only get to use subterfuge once a fight where as you get to use shadow focus at least 2 possibly more times. So its ~2 seconds of FW versus 2+ free ambushes.