You will find that over longer journeys the start up doesnt make that much difference... but in winter the car will run on higher fuel (choke) for a slightly longer period. Where as in summer it will be up to full operating temp in little or no time at all.
Not sure about the rest of the guys on here but the decrease in performance on a warm summers day is very noticable.. in fact if I remember correctly there was a discussion about it over on the MK5 board not so long ago.
I can well imagine that in winter, where I used to live, there were weeks when my car wouldn't warm up at all. (My neighbour who was an RAC patrolman said he thought I'd have trouble with my engine because of that, but it never happened.) That could well explain the much poorer economy at that time.
I've never been aware of a loss of performance in the summer though. On the other hand, I don't thrash my car. I don't think the Peugeot has ever been near the red line in its life.
So, if 98 really does improve performance, is there a case for using it in summer only?
Rolfe.