Top tier gasoline is ONLY 100% gasoline(without ethanol), E0. My last 5 cars obtained 8%, 8%, 7%-8%, 7% & 5% better mpg, when switching from E10 to E0. All cars run smoother, quieter & with a trace more low rpm torque. 87 octane E0 gasoline uses 87 octane gasoline. Duh!!! 87 octane E10, using 10%, 114 octane ethanol, must balance the octane with...... drumroll please...... with 84 octane gasoline molecules. Therefore, if your car knocks using 87 octane E10, it is probably the 84 octane gasoline molecules which are knocking. Switching to 87 octane E0, raises gasoline molecule octane by 3 points over 87 octane E10.
I think that Top Tier relates more to the detergent in the fuel to keep fuel and emissions working to top efficiency and not the ethanol percentage.
Local Chevron station has 4 grades of gas / petrol and all are classified as top tier. 87, 89, 91 all with up to 10% ethanol and 94 octane with no ethanol. add about 5 points for conversion to European ratings.
A couple of cars ago I owned a SAAB and it was noted that in more than one article that ethanol drove it crazy delivering up to 25% poorer fuel economy. Ironically in the owners manual (not hear say) while premium was recommended it was OK to use regular for short drives. The car would deliver about 50 HP less but give better fuel economy, very different from most other cars.