^^
As expected I have noticed a difference in power delivery at the top end, maybe not quite as eager as she used to be on 98.
MPG wise I have honestly not noticed any difference at all, if anything a slight improvement but this might have more to do with the weather than anything else as its gotten a lot warmer in the last couple of days.
I think the MPG benefit will depend on the type of journey you do. High octane is said to be better as it allows the engine to stay in lean mode for longer, but the engine will only go into lean mode if its at a steady speed and increase in power is not being demanded - i.e. cruising.
My commute is mostly town traffic and a little blast on a dual carriageway; so I doubt it gets the chance to go into lean mode anyway which is probably why my MPG hasn't really changed. Others doing motorway journeys may not agree ...