No, using an 800 kW motor on a machine that only loads it to 500 kW will make things run very much cooler which is usually a good thing (The motor will (steady state) only draw power equal to the mechanical load plus losses), but the over sized motor will:
- Run cooler, and therefore last longer all other things being equal.
- Potentially have higher starting torque (and starting current), you will need to allow for this in your motor starting arrangements, should not be a big deal if running from a VFD, might be if doing something old school.
- Have a lower winding resistance, hence a potentially higher locked rotor current, again an issue for protection design more then anything else.
- Possibly have a slightly worse power factor when run at 500 kW, so you might have to deal with a little higher kVA number when sizing wiring and controls.
However, when dealing with the thick end one of a MVA of power, probably at 11 kV or so I would not be listening to some random person on the internet, seek professional advice, this stuff takes no prisoners.