Why is premium 91 octane required in the 2011 Tiguan 2 liter engine?

Don't use octane booster, it won't make it 91 unless you buy a lot of it and then you can just buy 91. High octane is needed for high compression or forced induction engines. The manufacturer has tested the engine and found that it will knock with lower octane or their performance and fuel economy estimates are based on high octane. I would stick to the 91. Try Costco, much cheaper there.