Technically 91 is more efficient as it's less prone to uncontrolled combustion (aka detonation), but the idea is that if the car doesn't require 91 you'll just have more fuel that is unburnt. I'm pretty sure that's how octane works... now I can't speak for the tsi engines and if the ecu compensates for the difference in octane rating, but judging that the 170hp rating for the 1.8t was acheived using 91, I assume the ecu compensates since we can run 87. I've only done one take so far, got 483 (8.6 L/100km) but that was mainly because I had some fun with the new car. This tank trip computer says 7.8 but I'm guessing that's a little optimistic. Driving is anywhere from 50-50 or 60 city 40 highway
Higher octane fuel detonates/knocks at a higher pressure and temperature than lower octane fuel. It also burns more slowly than low octane fuel at the same temp and pressure.
On an engine designed for 87, higher octane is likely a waste unless pushing the engine to extremes - by that I mean towing up hill on an old engine or such. In that case, the engine is working so hard, chamber temps climb, and the ECU can't do enough by enriching the mixture and adjusting timing to cool the temps to prevent detonation/knock.
If the engine is made for 91 and you feed it 87, you might get away with it if gentle and/or accept poor fuel economy. The ECU will dump extra fuel into the engine to prevent knock/detonation - the extra fuel acts as an octane boost, if I remember right, by cooling the charge (evaporate some more fuel and the phase change sucks up a bunch more heat) and over successive strokes, by preventing full combustion (due too rich) also cooling the chamber temps.
So in a turbo vs a normally aspirated engine of the same design (pretend you add a turbo to a N/A engine), the turbo is cramming more air in, raising the pressure, and when a turbo compresses the air it heats it up, so raises the temps as well, thus requiring higher octane fuel to prevent detonation and make sure the flame front is controlled as well.
On a side note that doesn't really apply to our tiny car engines, if the octane is way too high, you can burn your exhaust valves... This can happen in the typically huge aero piston engines. There, if the octane is too high for the engine, the flame front hasn't burnt all the fuel before the exhaust valve opens and you blow fire past the fragile valve edges... These engines have huge pistons - damn near paint can sized vs the pop can sized pistons cars typically use.
Cheers,
Eric