Whether a new sleeping bag would help depends on your hammock setup. What do you use for insulation under you? Pad, underquilt, both? The under insulation is frequently the limiting factor. The bag won't do much to help there.

Phat is known to hammock at those temps with pads and a poncho slung under him as like a Garlington taco so it can be done. Myself I tend to go to ground when it gets below 20.