I am currently using all 100v rated components apart from the current shunt amplifiers which only go to 80v. There are no >80v shunt amplifiers avaliable so I need to put something around it to 'float' it at the common mode voltage.
I think this is the most useful app note on the issue http://www.ti.com/lit/an/sboa198/sboa198.pdf
The simplest method is to just add a resistor divider with precision resistors. So I might try that out and see just how bad it is.
Only problem i can think of is that those shunt amplifiers are designed for low impedance signal source.
There are optoisolated analog (operational) amplfiers. Those are specifically designed to amplify and pass on phase currents. Also there are hall sensor devices like the ACS711 (low voltage only) and bigger like the ACS758.