Well there's no free lunch, running a new car is an expensive business, although the VW residuals do ease the pain.
You'd probably get an R for less money than that GTI right now (you can lease for 2 years at about £6700 all in - a true £279 ave per month cost), with the difference more than making up for the extra fuelling costs between the 2 - something to consider. Once you get in the habit of regularly getting a new car, we can all consider ourselves renters really - no-one keeps a car for 10 years these days.
From a fuelling point of view i'm probably in a similar boat to you, coming from a GTD. I'm getting made redundant next year and there seems plenty of work in my field going right now, so i'm going to buy an R outright with a small part of my lumper. With £5k equity in my GTD at pick-up time and £3600 off RRP, i'm needing to put £22k in. If GFVs haven't been overstated, that car will be worth £19600 in p/x after 3 years, £2400 depreciation for me to suffer. With that in mind, fuelling costs are worth it to me for the extra fun factor in having that R. Those are my reasons to change.