Looking at the Testo 557 datasheet, it rates vacuum accuracy as +/- 1% FS (Full Scale) +/- 1 digit (at 22C).
This is a helluva way to rate a micron gauge! I don't know what they consider as "full scale", but, generally, when working with vacuum, I would consider FS to be 760,000 microns. Therefore, 1% FS would be 7,600 microns! In addition, 1 digit, represents 500 microns, as you have indicated.
From my vantage point, then, a reading of +/- 8,100 microns (at any vacuum level and at 22C) would be within factory spec. And no specs are given outside of the 22C temperature point. TIFWIW!
Full disclosure for those who don't know: I am the inventer, designer, and manufacturer of the AccuTools BluVac & BluVac LTE gauges.
Edit: Upon further reading of the datasheet, the vacuum range is 0 to 1 bar. So, one bar, or 750,064 microns, should be "full scale".