If I could offer some insight to turn 1 as I've been through it more times than I care to count in real life... The reason the car wants to twitch so bad is that the slight left entry into the corner causes the rear of the car to become neutral and unfortunately there isn't really a cure here. Downforce over the rear does help some but it's not all that matters. If you add more front brake the rear end comes around, if you add more rear brake the rear end still comes around, if you set the car up for oversteer on that track well good luck to you, if you set it up for understeer you'll plow right through turn 1. When I'm racing there in real life I will typically brake much earlier than GT5 tells you to, but only about quarter pressure just to get the car settled on the suspension for entry, right before I come off the "banking" of the tri-oval I'll usually quickly dial in a tad bit more rear brake between my downshift from 5th to 4th, usually only one click to the rear, sometimes two depending on tire wear. I guess the real key is to keep the car as straight as possible and go wider than you think you have to in order to keep a straight braking line, turning and braking don't mix especially from high speed.
If you think it's difficult in the game, try coming through the tri-oval and 300kph and having to brake down for the real turn, I'd say 99% of the guys out there, including myself could run a full second faster lap if turn 1 wasn't so terrifying.