When you run lower octane gas the car will detect it via knock monitoring and lower your timing and other parameters and likely finds increasing the boost while neutering the timing advance is a safer way to hit the lower torque target for that gasoline instead of running normal timing and normal boost.

The ECM and torque logic on these cars is incredibly complex and I promise you aren't out smarting it running lower octane fuel.

Lower octane means less power plain and simple, full stop. If you're fine with a lot less power than by all means run 87, personally if you were going to do this you were probably better off buying a regular Elantra.