We used to have a Smart car, that was remapped, from memory, to a heady 80bhp.
Optimax as it was then made a huge difference to the drive of the car, to the point I wouldn't put anything else in it.
Very noticeable, and I wouldn't say I'm a driver in tune with an engine well enough to notice an extra few % power on a more 'normal' car.
On that basis, I've always put 99ron Tesco or V-Power in my Alpina, as they recommend super unleaded only, but in theory it'll retard the timing to cope with standard 97ron.
I can't feel a difference between V-Power and 99Tesco.
Merc will get standard unleaded, but I try to avoid supermarkets - that Morrisons fiasco a while back made me paranoid about poor quality fuel.
Another point is I think fuel deteriorates with age, losing it's 'rons' - so if you have a sunny-day/once a week/month car, keep it very low on petrol and top up frequently, or use high grade fuel that may stay at or above 97ron for longer.
