I try to answer the question at what anode voltage we will see significant reduction of dE/dx resolution. 
In above plot I have summarized relative dE/dx resolution per pad row versus anode Voltage obtained in Run IX Tpc anode voltage scan.
The accuracy is  not very high (due to statistics) but I can conclude that the significant damage of dE/dx resolution will happens with Inner anode voltage below 1020 V and
outer anode voltage below 1220 V.