In my opinion, if the panel is manufactured and tested for the standard conditions then allowable load will be 100%rated.
See-for instance- IEC60439 Low-voltage switchgear and controlgear assemblies and IEC60694 Common specifications for high-voltage switchgear and controlgear standards.
Since a voltage unbalance is limited by motor standards the maximum unbalance it is stated in NEMA MG1/2009 Ch.12.44 voltage unbalance 1% or IEC 60034 Ch.7.2 Form and symmetry of voltages and currents as following:
"Three-phase a.c. motors shall be suitable for operation on a three-phase voltage system having a negative-sequence component not exceeding 1 % of the positive-sequence component over a long period, or 1,5 % for a short period not exceeding a few minutes, and a zero-sequence component not exceeding 1 % of the positive-sequence component."
