A thought occurs to me.

Gas for the power source but not just used as a rocket-type thruster.

Turbine.

Run the gas through a minisculeturbine to drive a tiny prop. Then exhaust the gas through the (hollow) propshaft.

Put a slight twist on the fins to give the torpedo a bit of stability when it's running.

Potentially twice the thrust for the same amount of gas.

BTW, I have no idea how you could acheive this, it's just an idea.