If you get the timing right, you could just shoot those at the enemy near the speed of light, and they would explosively decay right as they arrived. If the mass was low enough, they'd be effectively invisible, wouldn't they? Screw antimatter bombs, these would be pretty horrific. Instead of kilograms' worth of matter-to-energy conversion, you could have hundreds of tons worth. Wonder what that would be in megatonnage?

Other than the whole "you wouldn't be able to move it around" because unless you have a portable gravity projector, that thing isn't going anywhere: it is a gravity sink. Your enemies would have be lured into range, at which point, longbows will do a much, much better job =D

Yes, the amount of gravitational potential energy release is much higher for the same mass than you get from fusion. I’ve seen estimates of 10% - 40% of the mass-energy of the in-falling matter.

You “just” need a way to get the matter there and a way to capture that energy.