I don't think "cheap" enters in to the equation. People can talk money all day. It's what's in the bank that counts. If someone does a stop and go drive to work everyday, 5 days a week and can save about $20 a week, thatnumber adds up over time.
A loss of about 4 ponies is nearly negligible in my op. On the other hand, if running a lower octane will cause harm to the engine I would like to see factual data showing that
with the current GTI.
Opinions are great but, facts are better.
Taken from 2015 VW GTI Manual:
"Using the correct grade of gasoline is very important to help prevent engine damage loss, the recommended gasoline octane rating for your engine is listed on a label inside fuel filler, if unleaded Premium grade gasoline is specified for your vehicle, then Volkswagen recommends using, tOP TIER Detergent Gasoline with a minimum octane rating of 91 AKI (95 RON).
Information on TOP TIER Detergent Gasoline, please go to official Web site, the gasoline grades most commonly sold in the United States and Canada have following octane, ratings, which can usually be found filler pump, unleaded gasoline is available throughout the USA and Canada.
Do not take your vehicle to places where unleaded gasoline may not be available, you may use unleaded gasoline blended with alcohol or MTBE (methyl tertiary butyl ether), commonly, referred to as oxygenated fuels, if the blended mixture meets following criteria, blends of gasoline and methanol (wood alcohol methyl alcohol).
Anti-Knock Index (AKI) must be 87 higher, blend must contain no more than 3% methanol, blend must contain more than 2% co-solvents, blends of gasoline and ethanol (grain alcohol ethyl alcohol).
Blend must contain no more than 15% ethanol, blend must contain no more than 15% MTBE, many fuels are blended especially for winter or summer conditions, volkswagen suggests that you buy fuel at busy stations where the seasonal adjustment more likely.
Starting fluids can explode and cause run-away vehicle condition, never use fuel with an octane rating lower than 87 AKI/91 RON, never use leaded gasoline! Leaded gasoline will severely damage your vehicle's catalytic, methanol-blended fuels that do not meet the criteria listed above may cause corrosion."
https://carmanuals2.com/volkswagen/golf-gti-2015-fuel-35860
Of course if you have Stage tuned your car that is a whole different story.