The toluene idea is very viable for use as an octane booster. Modern gasolines have a large percentage of aromatics similar to toluene, around 15-30%. This is where they get alot of their octane rating. I think pure toluene has an octane rating of around ~116 or so? Maybe someone could correct me. If you run a 30% mix of 93 octane/toluene you will get 100 octane, and that would allow significantly increased compression. It is also cheaper if you buy it in larger quantities, and can be obtained for around $2.50/gallon, same cost as gas.
If you run that though you may need to re-calibrate your carb as it can change the specific gravity of the fuel.
Cars will never run worse with high octane than low octane. Running different fuel brings up issues as noted above. However, leaded gas will mess up your O2 sensor permanently if your car is fuel injected. And high octane does not say anything about burn rate, just resistance to detonation. It is due to the molecular structure of petroleum components, ie low octane has an abundance of straight chain hydrocarbons vs. high octane with alot of branched/ring hydrocarbons.
About compression vs. octane, it is a complex issue, primarily relating to compression chamber design. If you have an efficient chamber, especially one like the ford yates head, and run a small chamber with large quench areas and a reverse dome piston, you could probably run 11.5:1 on 93 octane. But, if you take an open chamber head and a domed piston, you would hit 10:1 and be pinging like hell due to total lack of quench area and poor flame travel.
This is certainly an interesting topic!
BTW I am Biochemistry/Organic chemistry major at UW-Madison, so if you have any other chemistry questions feel free and ask! I have to put my useless knowledge to work somehow....