I think 900 core is not possible with the stockcooler other than in artic conditions for more than just a single benchmark.

850 is quite easy and should be within reach with the stockcooler.

The reason for your bad score is most likely throttling, just go easy on volts with the stockcooler.

If you want to run this card over 850 core, you should watercool it.

Mine went from 59ºC idle to 32ºC idle with watercooling, a nice improvement i'd say.