I'm forced to quote you as the S4 did only slightly better than the usual Evo/Scooby hegemony for me on Expert difficulty. Bowie knife99 and his pals left me in the dust nine times out of ten.
I was a bit (well, a lot) of a stats slave in FH5 and was ready to do the same thing in 6 until
@breeminator reminded us all that the choice of car affects the performance and, in some cases, even the choice of which Drivatars the player's up against.
After trying every car I could get my hands on in B-class Retro Rally I found that this Renault 5 Turbo didn't bump up the AI speed much and turns on a 5-centime piece (I think they called it a sou in those days). Catching the opposition proved no problem for me as it really did le business, even at Takashiro Trail which ate my dinner in the other vehicles.
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I didn't try its compatriot Peugeot 205 T16 which although another superb handler is tuned to a different class in my garage.