I don't know what you guys have got against the XJ220! I took it through the Pro World Championship with no problems at all. The setup wasn't that difficult to crack to turn it from a power-off oversteer nightnmare to a nice, predictably handling race winner.
I know it's not in the same class as the GT-One when driven by a human but the AI driver of that machine never got closer than when it was sitting in spot two on the grid! And lets face it, the GT-One is nothing but a Le Mans monster rather than a legit road car like the Jag so it's more or less bound to be a stronger performer (not to mention the small matter of the game being slightly biased to make the Japanese cars seem better!).
I've not won the GT40 yet so I can't comment on how that compares to the XJ220. I think my Playstation senses I don't like rice burners and skews the odds so I keep winning GT-One's, R390 Road Cars etc! It took me six attempts at the World Championship before I won the Jag so I hope it wont take another six for me to win the GT40!