Overbanking in POKSY 757-200

You can change it in that aircrafts.cfg file. Just experiment with the section that mentions AP and the parameters in which it operates, with reference to the problem, i.e. does it happen in descents or climbs etc. Don't forget to back up your original 😉

I am being very tentative here. Does FSpax allow for the rather sensitive FDEs of the POSKYs?

From what I gather, FSP doesn't always have all the facts right about all the available aircraft.

This is very difficult to be managed.
FSPax works almost perfect (with the known bugs)for default FS crafts and for a big number of add-ons.