It depends on where the mass is. Mass ahead of the center of lift does increase stability. Mass behind the center of lift causes instability (not hard to figure this out if you give it some thought.)
You can try it yourself if you like: Make a paper airplane. Fly it, observe how stable it is. Put a penny in the nose, repeat. Move the penny to the rear, repeat. If the results are too extreme with the penny all the way forward or back, adjust it in between.
That's why my comment depended on the location of the engines: they're way back in the plane. That's dangerously close to the center of lift, which might be OK for a racer but would make long flights (with fuel mass changing, never mind munitions!) really dicey.