Well, you certainly don't run straight Xylene. That would probably be bad. I treat it like a gas additive, and I only do it sometimes (going to the drag strip, long highway trip in the heat - my car HATES that right now and I'm doing it next week).
In order to determine how much to put in, you use a weighted average. (x gallons Xylene * 118) + (y gallons pump gas * pump gas octane) and divide it by x+y (gas tank capacity). That'll give you your new octane. IIRC, 2 gallons of Xylene with a tank of 93 on top gives me 97 or 98 octane.
The only performance benefit is to reduce pinging/detonation. Otherwise, you are just burning expensive gas.
