I believe the Superbird was already defended once in this thread, but I'll say it again too; it's an alright car if you change your driving style to suit it. Brake incredibly early, and it gels a lot better. After I figured that out I was able to win that Mission with it.
As for the 787B, I'm shocked someone doesn't like it. I find it so incredibly easy to drive, yet it still feels realistic. The Minolta feels unfairly grippy, and the other cars from the Mazda's era (the Nissan R89/R92, Sauber, etc), not to mention the GT-one, feel a lot less stable. I'm sure full-tune the more modern cars post faster times, but the 787B is the only prototype I can take around the 'Ring at full tilt... while having a discussion with someone in the room, and still post a low 5min time. The only disappointment is not being able to give it an extra gear.
What dissapointed me about the SUperbird was the fact that it accelerated like one of those japanese microcars. I could never get it to top a hundred. I would expect a 400HP car to accelerate better than that.