Well, I didn't actually A-spec the Roadster Endurance Race, but I did pass this one cleanly (twice) last weekend.
I gradually found how to avoid losing speed so as to catch the other cars, and eventually carried just enough speed to get inside the other car in the last corner without getting a five-second penalty.
The first time I tapped the other car on the rear fender; not really clean since it moved it, but it wasn't hard enough a tap to trigger the penalty.
The second time I managed more of a door-handle-to-door-handle thing, once again gentle enough to get no penalty.
Prior to that I had discovered a different way to cheat even worse. I watched my old replay and I'd got first by slamming the AI car hard enough to send it off the track, so that it couldn't catch me even with my five second penalty.
But the other way to cheat is to simply head across the grass as soon as you see there's no barrier in the way. You end up on the other side going rather slowly, so you need to block vigorously to keep first, but it can be done. But why do that, really? (Except to demonstrate the foibles of the game). It is possible to get inside the other car thoroughly enough to get past without hitting it hard enough to incur a penatly.
I also redid
Mission 10, so now I own my Delorean legitimately.