I tested a couple tanks of 87 Octane in various driving conditions.
High speed cruising is easy, smooth and strong, never a problem.
The most demanding was climbing a long steep hill at low speed on 4th gear. Engine was stressed, weak and way overdriven, but it did not knock. So the engine works OK with 87 Octane. If I detect any knocking I will add some octane booster to the fuel tank, so far not necessary.
I am OK with losing some power. I don't race and never need to run that engine at full power anyway. For high speed cruise, at 4000 RPM the car would be running at 95-100 MPH. I never need to run that engine at full power up to 8500 RPM cut off.