Yes, peak combus pressure may be reduced but Octane rating is all about how COMPLETELY a fuel burns ie. high octane fuel=slower flame front =more complete combustion=greater thermal energy created =more power especially in a turbo engine(according to BP petrochemical engineers comment I found on Yank site)
There are so many other factors governing detonation ie. intake temp etc; not just dropping the comp ratio. This is why the Dahlback racing golf 4 (www.Dahlbackracing.se) can run on +-112 octane fuel @ 3,6 bars boost and a 9.8:1 comp ratio. This is all very well
BUT I still seek that optimum mix ratio for toluene to race gas, someone must know it?