850 seems a bit low for gig.. And you don't need 6 for gig, shoot you don't need 6 for over gig..
https://i.imgur.com/pGH0df6.jpg
First test is to nas 1gig interface from my pc, 2nd is to its 2.5ge usb interface.. Over cat 5e
Lots of things could come into play on why your not seeing 940ish, etc.. Security software on either end of your test. Driver, or driver settings, etc.. Some other sort of limiting on either of devices related to load, etc..
But normally yes you should be able to see 900ish for sure on a gig connection everything is running optimally.