Judging from the Gran Turismo PSP gameplay footage
here, Cote d'Azur still only has the original layout from GT3/4 as of E3, at least in the demo they used (pause at 0:53 in the video; there is only one layout available).
There could be a couple of explanations for this. Most likely, in my mind, is that they do not plan on designing an updated layout of the track for either GT5 or GTPSP. Less likely would be that they are planning to have a new layout for both games but it is either not complete or it was withheld from the demo for some reason. Least likely, in my mind, is that they are planning a new layout of the course for GT5, but not GTPSP, as I doubt that Polyphony would add layouts to one version but not to another, seeing as adding multiple layouts of the same basic track would probably save time and disk space (important on the PSP).
Naturally, different design decisions that I have not covered could affect the likelihood of any scenario, but that's the way I see it at present.
With regard to online racing and the possible complications due to griefers and poor driving (it's not necessarily one's intention to drive poorly, I should know

), I think it is a moot point, so long as private racing/room options are provided, as is generally expected. However, I don't recall having any trouble against the AI in GT4, and I don't believe that substituting real players for the AI would make it unplayable.