I think you provided some good insight but I don't think it is obvious and it needs some clarification. There is no "European" Gallon. Only Brittan uses the imperial gallon for some few remaining measurements. All gasoline is sold by the liter not the gallon. But my Octavia in addition to reporting liters/100 miles as is the standard European mileage measure also reported Miles / Gallon, and I am sure that you are right - that would be their imperial gallon measure and so give a number 20% better than ours.
As to the bumper weights and catalytic converters, the European Union Environmental standards are very similar to ours and I don't think this would be a significant factor.
Also higher octane fuel is not "better" and octane has nothing to do with fuel economy only on preignition depending on compression ratio.