[Luckypants]

You will not notice very much difference in an SV as it has no knock sensor to help adjust the ignition timing to take advantage of the higher octane of 97 RON. So there will be a negligible economy advantage, but not great.

However your GTi will have a knock sensor and will be able to adjust the ignition timing to take advantage of the higher octane rating, resulting in more complete combustion and possibly a decent improvement in economy. You will notice a difference in power and torque in the car.

My old V6 Omega loved 97 RON (the higher the better) and around town almost justified putting it in, but not quite. I think it improved from 17 MPG to 20 MPG with super plus stuff.

Final thing, I notice super is only a couple of pence dearer than plain old unleaded in my local Tesco. It's cheaper than diesel ffs! So I would defo try it in your GTi.