I won all the
Speedster races in B-spec. 4/5 were with a wing fitted with 0 downforce.

The wins weren't reliable, but I think if I did them again with downforce turned up they should be. (~107pts A-spec) Seattle was a complete walk-over. Afterwards I managed to A-spec them, having discovered that you need to raise the downforce on a cheat-wing.
Stats are 10,000/100/100/100
Passing still varies. It seems important to vary the pace and/or pass indicator or both. Switching to pace 5 seems to make it take an opportunity, but remaining in pace 5 it might not. Occasionally you can get it right and choose when to select the "passing line", too. But I tend to get more luck keeping pass on and switching between pace 4 and 5, mostly. Driving aids (ASM) can help with tricky parts of tricky courses. E.g.
Infineon.
Someone else says "pass on" when leading tends to make it block, too. I've seen things which tend to confirm that. The same person said that pace 5 is usually overdoing it, especially on corners. It seems mostly like that. "4 in, 5 out" works for a lot of corners. For a few slow corners, "2 in, 5 out" works, with the trick seeming to be to switch to 5 immediately after you hear the braking finish. I.e.
before you've actually entered the corner.
With faster cars, my B-spec driver is generally better than I am. I'm not really very good. That's why I trained
him so well. (Consolation prize).