Stealth - the yanks use a different method for octane ratings compared to the rest of the world. They use something called the 'anti-knock index' (AKI), whereas we use the 'research octane number' (RON). AKI is
supposed to be slightly more accurate at determining octane ratings when compared to RON - but in my VHO, the jury is out on this, if RON is good enough for Ze Germans, the Cheese Eating Surrender Monkies, along with Blighty's own . . . .
Because of this AKI and RON cannot be compared exactly. However, IIRC, yankie 87 AKI is equivalent to our 95 RON. The real difference for the shyteness of yankie go-go juice is their high sulfur content. Europe uses 'ultra low sulfur petrol' (ULSP) - hence why we can run our cars in lean-burn stratified mode, whereas the yanks can't. If they were to use lean-burn, their skies would turn a pungent orange - from the nasty sulfur dioxides.
You're spot on about the GTI needing 98 brew, and its quoted power. It also applies to fuel consumption too.