Ford Focus ST170?. If that was tuned up to the standards of the gymkhana rally cars then an F40 wouldnt even stand a chance, so it really depends, fully tuned cars are basicly racing cars at that point and if you can get enough power from the engine then sure its realistic. Though the AI never drives the cars to their limits, especially in the earlier stages of the game, so you can always get away with it.
I fully tuned a 97' Honda Civic Type R (400bhp, 850kg ish) to race in the expert level japanese supercar contest, against the Nissan GT-R (Which is far faster than a Ferrari F40) among other fast jap cars, and i was able to win without much trouble, though my Nissan GT-R is a lot faster than my tuned civic. The Ai just drive slow, they brake too early into corners.