There's loads on the web about the use of toluene and xylene for octane boosting. Both give similar effects. Typical website is:
http://www.vtr.org/maintain/gasoline-octane.html
A bit confusing 'cos it talks about octane according to the US definition (RON+MON/2). Anyway, to increase from 98 to 100 RON, you'd only need about 10% - 15%.
A few cautionary comments:
1/. it's illegal unless you pay the excise duty for a road fuel. Paint shops (which is where this stuff can be bought) will rightly refuse to sell you it if they know that you are using it on the road.
2/. May cause softening of old elastomers, hoses etc., so keep an eye on things and where appropriate replace with new.
3/. Don't get it neat on your paintwork.
4/. Large volumes of any single component hydrocarbon will upset the distillation characteristics, and these (aromatics) are both fairly high boiling point materials. May affect transient response a little bit, and may cause bore wash due to slow vapourisation. However, these would only be an issue with large volumes. 10% - 15% shouldn't be an issue, especially as the fuel standard from 1/1/2005 will further limit aromatics in retail gasoline.
Usual disclaimer - none of the above should be construed as any form of recommendation. If you do chose to try any of the above, it's at your own risk.