The only time I ever tached an OS 91 4 stroke was to compare it against my TT .91. My TT .91 beat it by about 700 rpm and tached out at about 10,100 rpm with a 14x6 APC running 15% coolpower.
www.rcfaq.com has a link for engine rpms and it appears 8500 is low for this engine. I am assuming you are leaning it to max lean rpm for a reading and then backing off a bit. If not, lean it to max lean with a tach, take a tach reading, and back off 300-400 rpm and you should be ok. 9100-9300 rpm is probably more in the ballpart for this engine at max lean prm.
If you have an older pre-surpass OS engine than this is probably normal rpm with a 14x6.