You're not being conservative enough.
First off, from everything I've read, the damage does not progress with rank, only the duration does. So now we're talking about 9d4. Secondly, it doesn't say 1d4 Rust damage and 1d4 Poison damage - it says 1d4 Rust and Poison damage. Thus it either needs badly to be reworded, or it's only doing the damage once but registering as two different types thus bypassing Poison resistance on WF and Rust immunity on everyone else.
So now we're looking at an average of 22.5 per tick. At rank 1 that's 67.5 damage, rank 2 is 135, and rank 3 is 202.5. And yes, it stacks, but with a 12 second cooldown the only way it will stack is if you're rank 3 and even then you will not get more than 2 stacks going. I hit harder than that with Takedown and that has a tenth the sp cost.
You're right about Tsunami though, it is pretty nice. But overall as a caster/melee hybrid I still find both Shiradi and Fury to be more attractive options.