hfg, I think the power issue is a bit more complex then that. The manufacturer specs are highly idealized, not sure exactly how they came up with such a small number.
Here is some data found in a review of the 840 pro:
Image
As you can see, during a heavy write workload it is quite possible to exceed or near 4W. Some SSDs are better, some are worse, and I think the Samsungs are on the better end of that scale, especially the 840Pro 512GB.
As far as thunderbolt power. I've been searching for exactly where in the standard it has a minimum power. The supply voltage that Thunderbolt provides can be spec'd as anywhere between 3.3V and 18V. It's really "up to 18V" and "up to 550mA" which makes it "up to 10W"? Can the Thunderbolt device specify the voltage and current it requires? And is the host obligated to provide it?
It is quite possible that some of these Thunderbolt HDD solutions request less than 10W, and/or can't dc-dc convert from 18V down to 5V. A quick calculation, if the device requests 5V (not having a dc/dc converter) and the Thunderbolt can only supply up to 550mA, that's 2.75W of available power.
Just a couple thoughts on the topic,
Mark.