First off, Go DOWN one turbo size. The larger the Turbo, the more revs you have to maintain to stay on boost. This effect is entirely too realistic in GT3.
You will find also the some chassis in the game can handle only so much HP. The Supra RZ is capable of getting 1000+ HP with mods. But it can only put about 600 on the ground EFFECTIVELY. With over 1000 Hp driving it is much like trying to push a rope.
As for your tranny woes: Boombexus gave up this trick a little while back.
1. set all gears as far to the left as they will go
2. Set Final drive as far to the right
3. Move the "auto-wide" slider to the right a click or two.
4. Test car. Run the track you want and if you can get to the top of the highest gear before you run out of straightaway. If you do get to the top of the highest gear before you get to the end of the straight, move the auto-wide slider 2 clicks further to the right.
5. Repeat step 4 until the car tops 6th gear at or near the end of the track's longest straight.
The Higher numerical final drive will give you added acceleration.