Robocop's translation sounds nearer the mark to me. I, too, think that we're talking about the quantity of energy here, rather than the size of the would-be crater. Hence the potential ambiguity in English pointed out by Lykurg doesn't exist from my point-of-view.
As another possible variation, how about ...?:
"... schätzt, daß der Einschlag eines Kometen oder Asteroiden eine Schlagkraft hätte, die fünfzehnmal größer wäre, als die des gesamten Atomwaffenarsenals der Welt."
(Hope I haven't made any grammatical mistakes there. It's a pretty tough assignment as far as punctuation and gender/case agreement in German goes!

)
Of course, however one translates it, it's a pretty silly statement that has little or nothing to do with reality (but which is probably fine for the comic/science fiction world that spohreis is interested in

). In particular, the factor surely depends on the size of the comet or asteroid, and probably no-one really knows how big the global stockpile of nuclear weapons is, either!
Cheers,
Abba