I incidentally dispute your efficiency claim anyway. I'd be willing to bet that if you collect every one of the bombs you were using for your Orion weapon and place them in single location then it would be more capable of slippiting the planet than the projectile would have been. Even if you managed to make it target the earth directly. If necessary you would of course use several years worth of the entire earth's production of steel (and maybe lead, gold and anything else hard or heavy) and use it to cover the bomb and keep the energy around a tad longer.

As I already said, the scaling laws mean the larger the bomb, the less efficient it becomes. Piling together 200 bombs just means you get inefficiency and possible fratricide from the explosion themselves or neutron emission. (Looking briefly at my old notes, underwater explosions have an exponent of tonnage raised to 2/3 for the resultant overpressure wave.)