OK, I just went and bought a nice blue Altezza RS200. Oil change brought it up to 210hp, plus I added Race I/E, NA/1, and the chip. This put the power total at 267. I bought (but did not install) the NA/2 power upgrade, leaving it in the trailer for now. I added T2 tires and Full Customize suspension. I set ASM and TCS both to zero.
A couple minor suspension tweaks off the top (increase front camber, lower car 10%, increase spring rates 20%) and I started the Series race. I did not qualify for any race; just launched off the rev limiter in 2nd gear. This combination was plenty for the first 3 races.
Midfield: +3.027" victory, fast lap 1':26.855", total 3':04.816"
R246: +2.646" victory, fast lap 2':01.436", total 4':13.720"
Deep Forest: +2.688" victory, fast lap 1':26.962", total 3':03.207"
For these 3 races I usually moved into 4th or so through the first few turns, knocked off 3rd and 2nd during the first lap, and picked off the leader somewhere in the last quadrant of the first lap.
Then, as you mentioned, the AI heats up. I ran R5 (a track I'm OK on) in the same car and could not get above 4th. Reloaded and added the NA/2 power but deleted the chip, bringing the total from 268 up to 294. That time I got 2nd, about 1.low" out. Reloaded again, added the chip from a total of 305hp, and squeaked out a win with a margin of less than a second. I then took Seattle without bothering to detune the car and again posted a larger margin of victory. Still DNQ.
SS R5: +0.high" victory, forgot to record times
Seattle: +1.967" victory, fast lap 1':48.987", total 3':48.367"
The AI gets faster and more aggressive at the last two tracks, without doubt. There are a couple tips to help you:
1) The Altezza in this form is a momentum car. Try to be smooth. Brake early and get back on the gas early and try to flow as best you can in the crowd. Hang back a little in the first part of the turn, and then you're already on the gas and gaining speed when they wash out, allowing you to squirt through on the inside.
2) Claim your turf on the inside, and defend it. Compromise your ideal line to stay inside a little, clear of traffic at the end of the straights. Come in a little hot and get right over to the inside rumble strip, because if they get inside you, they'll push you right out. Don't worry about the outside guys turning into your outside door - their fault, not yours. If they get a wedge between you and the apex, you'll lose at least one place on that turn.
3) Keep off the walls as much as you can. This car is not made of acceleration, and you need to conserve momentum.
If you're still having trouble, add the full custom transmmission. The gear spacing on the Altezza stinks - 2nd is too short and 3rd is too tall - and a tight-ratio 6 speed can get a lot more speed out of the car for the same power level.
Hope this helps!