Well, if you're talking about the power of the average Imperial I or II class Star Destroyer, then I don't think it's too far off the mark. I mean, they are damned inefficient, energy-wise, but they're still powered on small suns the size of large asteroids. And that a team of (I think it was) 3 of them is more than enough to glass your average M-class planet in an hour.
Their main problem was that there were shitloads of inefficiencies, design flaws, and outright weak points in the Imperial I & II's designs. Lots of places for smaller craft to hit, and whatnot.
They really should have just continued producing the Victory-class, and kept upgrading and updating them with the latest systems every decade or so.