Xylene is a C8 hydrocarbon, like isooctane. The octane value is ~115. It is an aromatic, like toluene, and is already in auto and aviation gasolines. The aromatic nature makes it a good solvent, so it is used as paint remover and dry cleaning. I'm pretty sure you can buy it at Home Depot in the paint department.
The octane boost of one gallon in a 20 gallon tank of 92 is about one number ( 1/20 times 115 plus 19/20 92 equals 93.1) I don't think it's a seal compatibility risk, I just don't think the benefit is worth the hassle.
I do think that 100% methanol is the best option. Besides the extremely slight octane boost, methanol has a very high latent heat of vaporization - it cools the surrounding by sucking heat out of it in order to get enough energy to convert to a vapor. That is why the highest power turbo engines (Cosworth XFE Champ Car) run fine - they actually inject methanol ahead of the turbos to cool the intake plenum, then also inject methanol at each cylinder as the fuel.
When the nurse wipes your arm with an alcohol to give you a shot, doesn't it feel cold? That's what methanol will do, too. Lowering intake temperatures by 10F just prior to combustion is almost a one octane number equivalent.