If you want to kill the AI(Tsukuba), the the Tuscan Speed 6 on RS tires will take the lead on lap 4, and have at least half the field a lap down, by the end of the race.
That's about what I had to do to get a win at Tsukuba
, I too tried the Spitfire first with a few more PPs (I think in the 480 range) and just was actually losing time on the lead Elise. Gave up a few times at about 6 laps and 40 seconds or more behind, just wasn't worth it, granted the car wasn't really tuned since I just stuffed some mods on it but still those Elises seem way too fast to me. Ended up burning it up with my TVR Tamora, mildly modded with Sports: Soft (I think).
Seems like it's one extreme to the other though, at London I had similar problems one time with my Spitfire and an Elise out front but then I switched to a TVR V8S (about 480pp or so with sports hard, I think, tires) and stomped all over the AI because there were only a few Elises and they got stuck behind some slower cars lol.
Maybe it wouldn't be as bad if PD didn't give the AI such a godawful lead from the get go, though I did laugh on the Mt. Aso track! I managed to get the lead fairly earlier then realised why, laughed so hard when I saw the 3 Elises I'd just passed come flying up over the last corner and spin each other all over the main straight
. That one's huge fun to drive on and I wish it could be saved for us to use online.