The thing is, the AI adjusts to your driving as well. Typically, if you're at the back of the pack, the AI slows down, but if you're dominating, the AI will speed up. I've noticed this in many races, and it's been tested by several people that post on other Gran Turismo forums. In a recent FF challenge I ran, I drove the Civic Type R and my lap time was around 1'32.xxx. When I drove the Integra Type R (same tires, both cars stock), my time was 1'28.xxx, but the second place AI car was always .5xx to 1.xxx behind me (Amateur League FF Challenge).
If you drive on Super Softs, it can give you at least a 2.xxx second advantage over normal tires. It's by far the best handling improvement you can make besides increasing downforce. By the way, what generates the extra downforce on the Treuno anyway? Does the hatch magically lift?
If you want a better matched challenge, race a Treuno SS with only an oil change, stage 1 turbo, and race intercooler ( I think it's like 280bhp) on normal tires in the Amateur 80's Car Cup at SSR5 against the AI Treuno SS. If I just suck and that isn't challenging enough, drop the stage 1 turbo and use a the race air filter upgrade in its place (approx. 260bhp off the top of my head).
Good driving, but I'm hard to impress with easier AI. Sorry...(and hear I am telling you to race Amateur mode.. ha!). If you try it, tell me what you think.