As far as impulse goes, the spring will probably be pretty inefficient relative to mass.

No. Very much no. The spring system would literally throw away half the mass of the craft for, maybe, a 10m/s delta. Fireworks would be more efficient. A pitching machine attached to a huge pile of baseballs would be more efficient (ie the baseballs could be thrown faster).

They all weigh a lot more than 1g though.

The point we are trying to make is that there hardly anything that would be less efficient relative to mass.