It's not just how much momentum is in the projectile, it's also how much energy is transferred to the target. A jumbo jet at cruising speed is likely to transfer almost all its energy into a large portion of virtually anything it hits. A small projectile traveling at 4500mph is likely to either transfer a lot of energy into a small area, or punch a hole straight through and keep on going until it hits something else.

Which isn't to say that it's not useful, but it probably isn't useful for the same applications. Something like this wouldn't be my top choice for large-scale destruction such as knocking down buildings. I sure as hell wouldn't want to be in a bunker or tank that has one of these things bouncing around inside, though.