To be honest, the stock tune will be more than good enough to win by a +30s. The only change I did, was putting it on RS tyres. The AI just doesn't really take advantage of the massive downforce, they're only "competitive" in the slow corners, where you're relying on mechanical grip. However when it comes to medium to high speed corners, you can just walk past them as if they're in a lower car class (if they don't push you of track or act like you don't exist). They're so slow through high speed corners, that they become a danger, especially in corners that are easy full throttle, they'll just brake in the middle of the corner for no apparent reason. GT AI has never really been able to cope with high downforce cars. The most difficult part about those events is not being taken out by the AI. If you try to go side by side through a fast corner, they'l most likely just push you into the barrier/off the track. Once you get used to them being rediculously slow you'll finish the race wih a 30 to 45s gap to 2nd place.