I used Supermarket petrol for 40 yrs until two years ago when prices were low and I had a long journey to make, I decided to try V-Power. Wow! It made a difference on my i-shift! 2nd gear seems to be more powerful. I get about extra 2-3 miles per tank more. They also give you voucher every quarter so it offsets the costs. As I m a driving instructor, sometimes due to location, I have to use supermarket fuel. The thing is my pupils notice the difference!
I got to say, V-POWER on Seat Ibiza doesn't seem to be a high difference compared to the Jazz.