The main problem with using additives is people chuck it in the car and hope for an instant result :/ you will only see benefits of adding toluene and such if you add it whilst on a rolling road tune to get the car to produce more power. The reason for this is toluene has some amazing anti knock properties so it will allow you to run more boost or timing before the on set of knock. It is supposed to work really well on turbo cars too producing more energy during the combustion and spooling the turbo a little quicker. Formula 1 used to run a mix of 85% toluene and 15% n-heptane which had a 0 ron rating. The n-heptane was used to stay in line with the rules to keep to the correct ron rating. That allowed them to run 75psi of boost on qualifying and 60psi in the race
but there are draw backs as if you mix it more than 40% you have to heat the fuel as it wont evaporate properly and will make your car run rough. Perfect in hot weather as it works well but in colder weather it won't work so well. Also I have recently brought some toluene and xylene to add to the tank but only whilst having it tuned I'm going to run 2 maps one with and one without. Its easy to source if you know where to buy think I brought 5 litres for around £20 so lots cheaper than most octane boosters you buy off the shelf