The obvious comparison point is metamagic rods. You generally need to be getting empower plus one of the other two to beat the cost of just picking up a pair of rods. The stones let you get two metamagics onto one spell, but you still need to get real value from the doubling or triping down to make it worthwhile. If one is enough, the rod is nearly always better. Unless you really need to grant a lot of druids the ability to empower a particular spell, anyway.
Not a bad setup, I suppose. Real problem might be the material component. The item demands that you cast this a lot, and 5k GP is a lot to ask repeatedly. Actually, real problem is it's a 9th. Generally hard to argue for reasonable efficiency upgrades when you're running 9th's. I do like the idea of setting up a weird tsunami defense system for a town or whatever, but it's kinda hard to imagine the threat that's totally equipped to bypass the 17th level druid or druids hanging out in this town on a permanent basis, which can't also handle a tsunami.Edit: Tsunami could benefit from all 3 effects. Extend (greater range), empower the d6/CL bludgeoning damage done to those caught up in the wave, and extend (you can concentrate on it for 2 rounds/level).
But Tsunami is not a spell you would usually want to use from the same location many times. Unless you are regularly defending your small island from invading fleets of ships?