I'm pretty sure the WP Fiesta has a 95RON engine:
http://www.drive.com.au/Editorial/Ar...leID=8105&vf=1
http://www.drive.com.au/Editorial/Ar...leID=8063&vf=1
Although reviews differ:
http://www.car4me.com.au/auto-review...Q~OpenDocument
It may be that the engine is less successful atr adapting to lower RON fuel.
My WS is zippy on regular 91 RON ULP provided revs are kept above 3000. One -just once - I went from a standing start, changed up to 2nd at 4000 RPM and floored it to the red line in 2nd to get away from some twirp. I was surprised how quickly 100 km/h came up - along with 6500 rpm (I was surprised). Slightly slower than our V6 Magna that does it in about 9.5 secs.
I have no doubt your car runs better on 98 RON vs 91. I question whether you would lose anything with 95 RON though.