as an avid hammocker and owner of a hennesey and other hammocks, I can respond.

First regardless of your prefered sleeping method, you should always use a pad of some sort.

on the ground you'll lose heat to conduction and in a hammock you'll lose it to convection.

Bags insulative quality come from the loft. once compressed, be it in a hammock or on the ground, that portion of the bag doesn't do what it was intended to do.

Second, I sleep exclusivly in a hammock when out. depending on the temperature, I'll use a vehicle windshield reflector for night temperatures down to 50f. For temps down to around 20f a thermarest pad.

Others have gone lower with both type of insulations, so your mileage may vary.

I suggest, before you depend on it, to practice and find out what is comfortable with you.

that being said, The best sleep I have ever had comes from a hammock