Kamelryttarn, with .040" quench I estimate you end up with 11.5:1 CR, cranking pressure about 255 psi.  With .060" quench I had about 11:1 CR and 245 psi, too high for pump gas here in Hawaii.

That 95% ethanol blend you mentioned would allow you to run very high CR, most likely around 15:1.  I agree with Armen that 18:1 is probably not doable.  Problem with alcohol is it's so darned corrosive.  It's hydroscopic and sucks up water like crazy.  Your carb and fuel tank will be a corroded mess in no time.  Additionally, you have to really increase the ratio of fuel to air to run straight ethanol.  You would probably end up with A/F ratio well below 10:1.  You will be washing down the cylinder.  I suggest you stick with gasoline.

I don't know all the specifics on European octane ratings, but here we use R+M/2.  We add the research number to the motor number and divide by 2.  So if I am not mistaken, 92 octane here  is equivalent to a higher octane rating in Europe.  I imagine your high octane fuel is probably at least as good as ours.  Once I installed the long winded cam with timing retarded, my tight quench engine ran good on the fuel available here.  But I always had concern over detonation, I knew I was on the ragged edge.  I was willing to run it like that because it made so darned much power.  I just got careless.  You can't be careless when you run on the ragged edge.

Hey Srin,
I was at Daytona in the 80s when the 5 valve Yamahas debuted. They were so fast! Funny thing, in a few years, they weren't. Everyone else took off and they did't get faster.
Kevin Cameron, my fav bike tech writer in the world, spent a lot of time ranting about what a bad design the 5 valve heads were for racing. Part of the problem was that with a high compression piston, the combustion chamber looked like an orange peel. Flame travel was so convoluted that the bikes were running 40 degrees of advance at a time when good 4 valve heads were running less than 30. Needing that much advance was a sure sign things were unhappy in combustionland.
Finally Yamaha ditched the design.
Wonder what other changes the PO made to your motor besides the high compression piston. Prob didn't do much.