Just came across this thread and thought I'd better add my 2 cents seeing as I have the Shamu.
I can vouch for it down to 5ºC (I think thats about 40ºf) while I was wearing lightweight polypro thermals and using my WM sycamore. I was plenty warm enough and could have gone a few degrees lower.
I pulled an edge of the shamu away from the bottom of the hamock while I was laying in it in the morning and the difference was huge, the cold air flowed straight in an the bottom of the hammock got instantly quite cold. Just stopping the wind hitting the hammock fabric makes enough of a difference plus the insultex's insulation qualities (hence the 'insul' tex