hence my accent on MORE, we just have a lower range and I couldn't remember if Lusankya was operating at 100% efficiency or not. here's a host of variables and the 1000 megatons is a very lower limit. Since we know how much energy goes into the shields we can determine if we have enough time how much energy it takes to destroy the Executor. Frankly, I don't want to spend the time doing that, but I figured that it's gonna take a hell of a lot of energy, basically this thing could probably go through a small star and remain intact.
As for your analogy, if you're using current day nukes, than you're most likely correct. However, they're likely to have developed higher yield warheads or more effect shaped charges. Both could change that analogy.