Toluene and xylene in fuel are usually only bandaids for high comp engines.

The calorific value of both toluene and xylene is low so when you use them to prevent detonation they also reduce the calorific value of the fuel. This means the peak combustion pressure is reduced and the mixture burns slower preventing detonation. The cost of this detonation resistance is reduced power due to the lower enrgy produced by each bang of the cylinder.

To make the same power as unadulterated hi octane fuels, you need more oxygen and more fuel to be burnt in the same time. This fuel mix is ideal in a turbo car where the amount of fuel burnt is easily controlled by bost presssure.

Often,a better solution is less compression and better finishing work on the combustion chambers and fire channel of the head. Good chamber design can make an engine burn more fuel efficiently and achieve better cylinder fill than mega compression. Another bonus of the lower compression engine and the resulting increased chamber efficiency is the reduced cylinder pumping losses from the engine.

There was a lot of work done on this subject in the 1920's - 1930's by Ricardo. He was the father of modern engines and fuels. His main work was done on compression ratios and how this effects fuel burn. This resulted in the development of knock ratings for a wide variety of fuels.

If you ever get a chance to read about him, (Ricardo) do so. It is an insight into the world of the engine design gods, and so much of what he found is very relevant today.