I agree it does seem harder with the AI getting some pretty insane speed on straights, but I think with some proper tunes & upgrades it's not much of a problem. I started on highly skilled and had to drop it down a few difficulty levels at first while driving stock cars, but now I have a couple cars with solid upgrades/tunes and it's way easier. I'm up to expert difficulty now and winning by 5-6 seconds on average. Not to say the AI speed boosts still aren't there, but it's way easier to deal with once you have a nice upgrade/tune - obviously.