But it’s not a static, fixed environment - you still have a mixture knob.
Piston engines measure fuel by volume, not weight or energy content. The carb (or fuel-injection) presumes each gallon of fuel weighs the same. So if you feed the system with fuel of higher density, in effect you’re putting more fuel in and getting more power out.
This means if you lean to a specific fuel flow, you’ll find yourself going (slightly) faster on G100UL than 100LL.
If you lean to a specific EGT or speed, then you’ll find yourself burning (slightly) less fuel while cruising at the same speed.