Last weekend, the only hanging spot I could find in a very scrub-ish offtrack campsite were two trees about 2 metres further away that I normally use. Because of this, even when the support lines were tied higher up the tree and tighter than normal, I found it difficult to adjust the sag normally. I ended up having to retie the hennessy style knot about 3 times. If the hammock were adjusted with prussiks, it would have been far easier.
As to the 8 mil rope, I'm sure its possible to use something a little smaller...