Thats 1100 meters.
Plymouth in the UK is 1160 meters and with a 30min trip with IFR reserves etc it was limited to 8 blokes with 10kg of baggage. Every +5 above ISA you drop a pax. APR and water meth won't help until your above ISA +5 because you will be torque limited.
Also as well its very airframe dependent how heavy it is.