for the A spec, I use a stock Toyota GT-86 from the new car dealership. It starts off around 400pp, you might have to add some ballast or set it to 90 something % engine power to get to 400, but it is pretty easy, when they are new. I then take the same car, get rid of the ballast and put engine power to 100%- which gives you 405pp-415pp (depending on how well the car is broken in) and I smoke the 500pp cars in that race with my approx. 410pp GT-86. The only problem I have when using this is getting passed on long straights by cars with bigger engines (i.e Skyline GT-R, NSX etc.) but I make up the time in the corners and have never lost a race on the last lap or last straight, I am usually 10+ seconds in front of the second place car on the final lap. I would not use the 410pp GT-86 in B-spec unless I had a BOB of level 20 or higher though, they make too many mistakes that are hard to recover from.
I have also bought a second GT-86, and did every possible modification on it, and it sits at around 530pp, then I de-tune it down to 500pp (or just tune it up to 500pp in the first place, no need to do all weight savings), and it smokes the 500pp no problem. I even made the mistake of running my new, never raced before level 0 BOB in a 500pp race with the highly tuned GT-86, and it had no problems getting first.