I don't know of any helicopters that have more than 10 seats that aren't Part 29 (or it's older equivalent).
You should be aware in those conditions that the HV curve is a limitation, and that this will seriously affect your operation.
Also, there may be WAT charts in the limitations section that are (obviously) limitations.
On the other hand, one of the reasons why more payload than expected might be carried is because the engines are above spec engines, and dispensation for this has been given.
Unfortunately, all too often the performance charts (and sometimes limitations) are ignored. Good luck sorting it out!