Interesting. I have a set of ARC-8's as well, running 9.5 and 10.5 265/285 PSS tires. I've been thrilled other than a vibration that manifests exactly as the OP has described. Very minor, and starts from 75 MPH -to low 80's. I have yet to go the route of determining the cause, though when they were mounted there wasn't any runout to speak of and everything road force balanced under 25 lbs IIRC.
No spacers and tires were mounted brand new.