To get back to the OP's original question 
No I don't think it will be night and day.
I don't think there is much in it. The power is delivered in a different manner in both.
When I was using my wifes GTI for a couple of weeks I felt it was slower off the line than the GTD but this was partly due to trying to drive the GTI like a GTD. It took me a while to change. All sorts of Kia's and Citroen's were hammering me at the traffic lights
However once I got used to using all the revs in the petrol it was slightly better.
I liked the free revving of the GTI and felt it was slightly more nimble than the GTD.
... so the GTD pushes you in the back from low revs, but has a shorter rev range?
... and the GTI requires more revs to push you in the back, and has a longer rev range?
... so for daily driving the GTD gives lots of shove with less effort?
... the GTI requires to be 'driven', but will give slightly better performance below 60 when 'driven' and much better performance at naughty speeds?
I had a GTI for a short test drive recently and did think it was a bit dead. It was brand new and I was just testing the DCC suspension feel - which I really liked. I noticed that for the same amount of pedal movement as my GTD the car just didn't have the same 'go'. I didn't try to use the higher rev range. The car was not 'run in'.
I'm planning on changing my car this year. After a lot of thought I have decided to put the GTI PP at the top of my list. Closely followed by the GTD.
Last week I drove my Mk6 GTD to the supermarket 7 miles away. It had not even warmed up.
My reasons for changing to GTI...
- Always wanted a GTI
- I'm doing less miles = not good for a GTD
- Want a tow bar - for a kayak trailer
- Can run on any petrol
- warms up faster
- I don't think I would notice much difference moving from GTD to GTD
- smoother, quicker, lighter, better handling, iconic
- really interested in trying the VAQ diff