99 percent of people will use 95 in a gti, it's fine, 97 is more than adequate unless mapped for 98. 
Completely disagree. Did you just pull that statistic out of thin air? Any proof to back that up?
I've done 500 miles on 95 RON, averaged 33mpg, I guess I'll just have to try 97 RON for the same period of time and see if it makes a difference or not.
As mentioned, of course you can run your car on the low octane stuff but it will retard the ignition timing when detonation is detected.
Interseting article from the interweb....
Many high-performance engines are designed to operate with a high maximum compression, and thus demand fuels of higher octane. A common misconception is that power output or fuel efficiency can be improved by burning fuel of higher octane than that specified by the engine manufacturer. The power output of an engine depends in part on the energy density of the fuel being burnt. Fuels of different octane ratings may have similar densities, but because switching to a higher octane fuel does not add more hydrocarbon content or oxygen, the engine cannot develop more power.
However, burning fuel with a lower octane rating than that for which the engine is designed often results in a reduction of power output and efficiency. Many modern engines are equipped with a knock sensor (a small piezoelectric microphone), which sends a signal to the engine control unit, which in turn retards the ignition timing when detonation is detected. Retarding the ignition timing reduces the tendency of the fuel-air mixture to detonate, but also reduces power output and fuel efficiency. Because of this, under conditions of high load and high temperature, a given engine may have a more consistent power output with a higher octane fuel, as such fuels are less prone to detonation.
And taken from
AUTOCAR magazine...
High octane equals better mpg
The benefits of high-octane unleaded petrol outweigh the extra cost of the fuel, expert tests have concluded.
Thorney Motorsport compared two identical Vauxhall Astra VXRs on the same routes over a four-month period.
Standard Tesco unleaded was used in one, Tesco's 99 Octane brand in the other.
The car running 99 Octane averaged 25mpg during the tests, while the lower-octane fuelled car returned 22mpg. That's an overall improvement of 14 per cent.
Daily dyno tests also revealed that the VXR running on 99 Octane produced 252bhp on average, compared with standard fuelled car's 235bhp.
Thorney Motorsport concludes that it's worth paying extra for the Tesco 99 Octane fuel, because you can go further on a tank.
However, high-octane unleaded is known to have a greater effect on heavily tuned and turbocharged cars like the VXR than it does on lower-spec, naturally aspirated models.
I'm sure that with a quick search a contradictory article can be found that's equally convincing but I for one will only run my tuned car on 99 RON. VW recommend above at least 98, JKM recommended me 99 and most peeps on here have recommended 99.
