Braking too early is something I think can be fixed. I don't know about the cornering speeds though.
If the AI was completely scripted, then yes, they could be crazy fast. But then they wouldn't be AI. And if they were ever bumped or moved out of the way, they would become useless.
True AI is obviously the way to go for a game like GT, but then you can't directly control how good the AI will be, so to speak. Right now, the AI operates in such a way that it never overloads the tires (at least by my observations). Now if the AI did push the car to the limit more often, they could be a lot faster. But would PD be able to code AI that pushed the car that hard, yet remained in control? I don't know, I'm not an expert on AI, but it seems to be something that everyone struggles with.
The only truly deadly racing AI I've seen use rubberbanding or "cheats". They've always kind of spoiled the experience though. However, I guess having them as options may not be too bad.
If you're OK with the Espace, you're OK with engine swaps. There is no difference.
You're dead wrong. First, you're trying to pass opinion as fact. Secondly, depending on how one defines the purpose or goal or whatever of GT, you can easily make an argument for the inclusion of engine swaps. You could do the same against engine swaps, but only if you used some narrow, arbitrary, definition of the essence of GT.
If engine swaps are useless, so is tuning. Why tune the Viper SRT-10? We have the ACR. Why add power to the F430? We have the Scud. Your argument is senseless, and I'd imagine it boils down to unreasonable favoritism of GT over Forza and other games. In other words your only reason to be against engine swaps is because GT didn't do it first, and of course GT can do no wrong, so leaving out engine swaps must be right. That's not a reason.