Need Help with Mclaren F1

[Schwartz]

was looking for a challenge, so I bought a Mclaren F1 to tune. I have upgraded it to 620hp and did all weight reductions. I am tuning it on sports soft tires. I have a few questions/problems
1: Is it stupid to tune the F1 on sports soft? Should I use racing tires?
2: Massive understeer. I dont have this problem on racing tires but when I turn the wheel (or analog stick in this case) in higher speed corners, the car is sometimes almost completely unresponsive and I go off the track.
3: The back end wants to slip out in low speed corners.
4. The brakes on the front wheels seem to lock up my tires when braking hard, even with ABS

I have tuned the suspension/LSD/brake balance to hell and back, and nothing seems to be working.
I need help!!!

 

[MrGrado]

You can use sports tyres if you like.

this will have Mclaren F1 tunes to try.

If you want specific help with you tune, post your setup.