45p a mile isn't all for fuel, it's for miles on the car and tyre wear, contribution towards having a service sooner because of those miles etc the whole cost of running a car. For me i'd be reimbursing myself 20p per mile for fuel with a GTI and saving 25p per mile to grow into a lump sum towards the next car when additional mileage accrued for business purposes eats into your p/x value (unless business miles are pretty small over the year).
I see that now, brings it down a little as some is for wear and tear - always had a card so never claimed so was not aware of that, another thing learnt 
Personal minefield these days making a decision!
Was commuting Cambridge to Durham each week for a while and got 50p per mile back, mostly for depreciation.
Anyway, with the new variable cam timing the GTI should be able to make good use of high octane fuel to get better efficiency and performance. From that article posted a while back it seems without this tech the high octane has less effect.
To the posts way back calculating GTI fuel cost vs GTD to assume 33mpg or similar for the GTI is loading the deck against it. I'll get 40 minimum without driving all that sensibly or I'll eat my hat! I get this on my current car which is much older, with a smaller engine, slightly worse efficiency and only 5 speed (I commute on A14 so the GTI will be much more suited to me, no more running my engine hard).
