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.