Re: So how LESS powerful is the Xindi weapon than 24th century Phasers
The strange practice of revising casualty figures upwards isn't dependent on the type of attack, it seems. The same thing happened with the Breen strike.
Nobody does it like that today: one always assumes the worst-case scenario first - everybody who cannot be reached is supposed dead or at least "missing", and things get happier when contact is eventually made with survivors. But that's a practice based on pessimism, a certain desired type of psychological impact, and a lack of any real means to judge the number of dead. The 22nd-24th century Earthlings might have learned how to predict or model the casualties of a given type of disaster, and usually the model would be smack on, requiring no revising. In the very rare case, such as with these two explicitly unprecedented types of attack, it would be revised upward.
Anyway, the first Xindi beam seemed pretty much like what one would expect of a 24th century starship that is left to work its evil unopposed. Just imagine the drilling beams of the E-D, as seen in "Legacy" et al., dragging along the surface of the target planet instead.
The second was significantly more powerful than any beam weapon we have seen elsewhere, though. But perhaps that's still normal 24th century phaser technology - perhaps a phaser emitter that is a kilometer across could be constructed by 24th century Starfleet to achieve the same effect, but it's not tactically practical to do something that big and clumsy.
As for planetary shields, I don't think the Feds have ever had anything comparable to that. The only large shield we hear of would be the one around Elba II, and that wasn't said to be resilient to starship weapons. It only blocked transporters - and it was said that if Scotty had chosen to fire at it to remove the blockage (supposedly at the minimum setting required), the result would have been a destroyed penal colony and a dead Captain Kirk, suggesting the shield wasn't worth anything as a defensive measure.
So they'd have to go beg from the Aldeans - but even then, all they would get would be an invisibility screen. And the enemy wouldn't need visible cues to attack Earth. An accurate almanac would suffice...
Timo Saloniemi