Switched to the shapeshifter now.
Using the runeflesh armor on him to buff his bloodlust skill as you suggested.
If i had to guess i would say that this bloodlust now equals roughly a lvl 20 bloodlust skill.
It is very useful, but less good than for example the lvl 36 proc from arreats heart.
Some kind of sheet about which lvl of skills mercenaries are using would be nice.