I'm thinking the 'average' GTI driver doesn't do anything like 24k a year - I've only done 9k in 14 months, for instance.
24k was what the original poster reported. But you're absolutely right about 24k definitely not being the 'average' -- your mileage would give around £215 difference per year (3+ full tanks), so while not breaking the bank, still measurable.
I think it's fairly safe to say that if you're driving a GTI, fuel economy isn't your primary concern. However, for those who do care even a little bit about fuel economy, then it is useful to know that some tyres are more fuel economical than others, and if all other things are roughly equal (i.e. same price and performance), then why not buy the more fuel economical tyre?
And speaking of all things being roughly equal, it's been a long time since I've researched such an emotive topic. "Best tyres I've ever owned" and "absolute rubbish tyre" said about the same tyre; "really noisy" and "much quieter than my previous tyres", etc. etc. The magazine reviews are inconsistent (hardly scientific) and the conclusions rarely match up with the individual results. Overall a difficult subject to research!